diff --git a/assets/js/phoenix_live_view/dom.ts b/assets/js/phoenix_live_view/dom.ts
index 5fa8dfa80f..7fc0a89752 100644
--- a/assets/js/phoenix_live_view/dom.ts
+++ b/assets/js/phoenix_live_view/dom.ts
@@ -696,6 +696,21 @@ const DOM = {
     );
   },
 
+  /**
+   * Returns true if the element is a Form-Associated Custom Element (FACE).
+   * Unlike native inputs, FACEs store their value in ElementInternals (via
+   * setFormValue), not in DOM attributes or children. Their light-DOM
+   * children and slotted elements must be patched normally by morphdom even
+   * while the element has focus.
+   */
+  isFormAssociatedCustomElement(el: Element | EventTarget | null): boolean {
+    if (!(el instanceof HTMLElement) || !el.localName) return false;
+    const ctor = customElements.get(el.localName);
+    return (
+      !!ctor && (ctor as { formAssociated?: boolean }).formAssociated === true
+    );
+  },
+
   syncAttrsToProps(el) {
     if (
       el instanceof HTMLInputElement &&
diff --git a/assets/js/phoenix_live_view/dom_patch.ts b/assets/js/phoenix_live_view/dom_patch.ts
index c52e44a585..a6bd1563a0 100644
--- a/assets/js/phoenix_live_view/dom_patch.ts
+++ b/assets/js/phoenix_live_view/dom_patch.ts
@@ -474,6 +474,19 @@ export default class DOMPatch {
             fromEl.type !== "hidden" &&
             !focusedSelectChanged
           ) {
+            // Form-Associated Custom Elements (FACEs) keep their value in
+            // ElementInternals, not in DOM children. Their light-DOM
+            // children and slotted elements must be patched by morphdom
+            // even while the host has focus. Returning the element instead
+            // of false tells morphdom to descend into children normally.
+            if (DOM.isFormAssociatedCustomElement(fromEl)) {
+              this.trackBeforeUpdated(fromEl, toEl);
+              DOM.mergeAttrs(fromEl, toEl);
+              DOM.syncAttrsToProps(fromEl);
+              updates.push(fromEl);
+              DOM.applyStickyOperations(fromEl);
+              return fromEl;
+            }
             this.trackBeforeUpdated(fromEl, toEl);
             DOM.mergeFocusedInput(fromEl, toEl);
             DOM.syncAttrsToProps(fromEl);
diff --git a/test/e2e/support/issues/issue_4323.ex b/test/e2e/support/issues/issue_4323.ex
new file mode 100644
index 0000000000..90c4e50080
--- /dev/null
+++ b/test/e2e/support/issues/issue_4323.ex
@@ -0,0 +1,112 @@
+defmodule Phoenix.LiveViewTest.E2E.Issue4323Live do
+  use Phoenix.LiveView
+
+  def mount(_params, _session, socket) do
+    socket =
+      socket
+      |> assign(:counter, 0)
+      |> assign(:render_in_root, fn assigns ->
+        ~H"""
+        <script>
+          // Case 1 & 2: Basic FACE, no shadow DOM
+          class FaceBasic extends HTMLElement {
+            static formAssociated = true;
+            constructor() { super(); this.attachInternals(); }
+          }
+          customElements.define("face-basic", FaceBasic);
+
+          // Case 3: FACE with shadow DOM + slot (no delegatesFocus)
+          class FaceSlotted extends HTMLElement {
+            static formAssociated = true;
+            constructor() {
+              super();
+              this.attachInternals();
+              this.attachShadow({ mode: "open" });
+              this.shadowRoot.innerHTML = "<div><slot></slot></div>";
+            }
+          }
+          customElements.define("face-slotted", FaceSlotted);
+
+          // Case 4: FACE with shadow DOM + delegatesFocus
+          class FaceDelegates extends HTMLElement {
+            static formAssociated = true;
+            constructor() {
+              super();
+              this.attachInternals();
+              this.attachShadow({ mode: "open", delegatesFocus: true });
+              this.shadowRoot.innerHTML = '<input type="text" />';
+            }
+          }
+          customElements.define("face-delegates", FaceDelegates);
+
+          // Case 5: Non-FACE custom element
+          class NonFaceEl extends HTMLElement {}
+          customElements.define("non-face-el", NonFaceEl);
+
+          // Case 6: FACE with shadow DOM + slot + delegatesFocus
+          class FaceSlottedDelegates extends HTMLElement {
+            static formAssociated = true;
+            constructor() {
+              super();
+              this.attachInternals();
+              this.attachShadow({ mode: "open", delegatesFocus: true });
+              this.shadowRoot.innerHTML = '<input type="text" /><div><slot></slot></div>';
+            }
+          }
+          customElements.define("face-slotted-delegates", FaceSlottedDelegates);
+        </script>
+        """
+      end)
+
+    {:ok, socket}
+  end
+
+  def handle_event("inc", _params, socket) do
+    {:noreply, assign(socket, :counter, socket.assigns.counter + 1)}
+  end
+
+  def handle_event("validate", _params, socket) do
+    {:noreply, socket}
+  end
+
+  def render(assigns) do
+    ~H"""
+    <form id="test-form" phx-change="validate">
+      <%!-- Case 1: Basic FACE, focusable via tabindex --%>
+      <face-basic id="case1" tabindex="0">
+        <span id="case1-child">count:{@counter}</span>
+      </face-basic>
+
+      <%!-- Case 2: FACE with light DOM input child --%>
+      <face-basic id="case2" tabindex="0">
+        <span id="case2-child">count:{@counter}</span>
+        <input id="case2-input" type="text" name="case2_input" />
+      </face-basic>
+
+      <%!-- Case 3: FACE with slotted input --%>
+      <face-slotted id="case3">
+        <input id="case3-input" type="text" name="case3_input" />
+        <span id="case3-child">count:{@counter}</span>
+      </face-slotted>
+
+      <%!-- Case 4: FACE with delegatesFocus --%>
+      <face-delegates id="case4">
+        <span id="case4-child">count:{@counter}</span>
+      </face-delegates>
+
+      <%!-- Case 5: Non-FACE custom element --%>
+      <non-face-el id="case5" tabindex="0">
+        <span id="case5-child">count:{@counter}</span>
+      </non-face-el>
+
+      <%!-- Case 6: FACE with slot + delegatesFocus --%>
+      <face-slotted-delegates id="case6">
+        <input id="case6-input" type="text" name="case6_input" />
+        <span id="case6-child">count:{@counter}</span>
+      </face-slotted-delegates>
+
+      <button id="inc-btn" type="button" phx-click="inc">Increment</button>
+    </form>
+    """
+  end
+end
diff --git a/test/e2e/test_helper.exs b/test/e2e/test_helper.exs
index 5c48cc2cf4..7ce0e96d10 100644
--- a/test/e2e/test_helper.exs
+++ b/test/e2e/test_helper.exs
@@ -218,6 +218,7 @@ defmodule Phoenix.LiveViewTest.E2E.Router do
       live "/4212", Issue4212Live
       live "/4290/a", Issue4290.ALive
       live "/4290/b", Issue4290.BLive
+      live "/4323", Issue4323Live
       live "/4325", Issue4325Live
     end
   end
diff --git a/test/e2e/tests/issues/4323.spec.js b/test/e2e/tests/issues/4323.spec.js
new file mode 100644
index 0000000000..e5e5c5ade5
--- /dev/null
+++ b/test/e2e/tests/issues/4323.spec.js
@@ -0,0 +1,118 @@
+import { test, expect } from "../../test-fixtures";
+import { syncLV, evalLV } from "../../utils";
+
+// https://github.com/phoenixframework/phoenix_live_view/pull/4323
+//
+// Form-Associated Custom Elements (FACEs) store their value in
+// ElementInternals (via setFormValue), not in DOM children. When a FACE
+// host has focus, morphdom should still descend into its light-DOM children
+// to patch them normally. These tests cover the fix and verify no
+// regressions for related scenarios.
+
+const incrementCounter = async (page) => {
+  await evalLV(
+    page,
+    "{:noreply, Phoenix.Component.assign(socket, :counter, socket.assigns.counter + 1)}",
+  );
+  await syncLV(page);
+};
+
+test.describe("FACE children patching", () => {
+  test.beforeEach(async ({ page }) => {
+    await page.goto("/issues/4323");
+    await syncLV(page);
+  });
+
+  test("FACE host focused: light DOM children are patched", async ({
+    page,
+  }) => {
+    await expect(page.locator("#case1-child")).toHaveText("count:0");
+
+    await page.locator("#case1").focus();
+    await expect(page.locator("#case1")).toBeFocused();
+
+    await incrementCounter(page);
+
+    await expect(page.locator("#case1-child")).toHaveText("count:1");
+  });
+
+  test("FACE host focused with input child: children are patched", async ({
+    page,
+  }) => {
+    await expect(page.locator("#case2-child")).toHaveText("count:0");
+
+    // Focus the FACE host, NOT the input inside it
+    await page.locator("#case2").focus();
+    await expect(page.locator("#case2")).toBeFocused();
+
+    await incrementCounter(page);
+
+    await expect(page.locator("#case2-child")).toHaveText("count:1");
+  });
+
+  test("FACE with slotted input focused: input is protected, siblings patched", async ({
+    page,
+  }) => {
+    await expect(page.locator("#case3-child")).toHaveText("count:0");
+
+    // Focus the slotted input (not the FACE host)
+    await page.locator("#case3-input").focus();
+    await expect(page.locator("#case3-input")).toBeFocused();
+    await page.locator("#case3-input").fill("user-typed");
+
+    await incrementCounter(page);
+
+    // Input value is preserved by normal focus-skip
+    await expect(page.locator("#case3-input")).toHaveValue("user-typed");
+    // Sibling children are still patched (FACE host is not focused)
+    await expect(page.locator("#case3-child")).toHaveText("count:1");
+  });
+
+  test("FACE with delegatesFocus: light DOM children are patched", async ({
+    page,
+  }) => {
+    await expect(page.locator("#case4-child")).toHaveText("count:0");
+
+    // Click the FACE host; delegatesFocus sends focus to the shadow input,
+    // but document.activeElement still returns the shadow host
+    await page.locator("#case4").click();
+    const activeId = await page.evaluate(() => document.activeElement?.id);
+    expect(activeId).toBe("case4");
+
+    await incrementCounter(page);
+
+    await expect(page.locator("#case4-child")).toHaveText("count:1");
+  });
+
+  test("FACE with slot + delegatesFocus: slotted children are patched", async ({
+    page,
+  }) => {
+    await expect(page.locator("#case6-child")).toHaveText("count:0");
+
+    // Click the FACE host; delegatesFocus sends focus to the shadow input,
+    // but document.activeElement returns the shadow host
+    await page.locator("#case6").click();
+    const activeId = await page.evaluate(() => document.activeElement?.id);
+    expect(activeId).toBe("case6");
+
+    await incrementCounter(page);
+
+    // Slotted light DOM children should be patched (FACE branch descends)
+    await expect(page.locator("#case6-child")).toHaveText("count:1");
+  });
+
+  test("non-FACE custom element: children are patched normally", async ({
+    page,
+  }) => {
+    await expect(page.locator("#case5-child")).toHaveText("count:0");
+
+    await page.locator("#case5").focus();
+    await expect(page.locator("#case5")).toBeFocused();
+
+    await incrementCounter(page);
+
+    // Non-FACE elements are not treated as editable inputs,
+    // so morphdom patches them normally regardless of focus
+    await expect(page.locator("#case5-child")).toHaveText("count:1");
+  });
+});