diff --git a/src/Type/Constant/ConstantArrayTypeBuilder.php b/src/Type/Constant/ConstantArrayTypeBuilder.php
index f4e96ad8894..907d49eb40b 100644
--- a/src/Type/Constant/ConstantArrayTypeBuilder.php
+++ b/src/Type/Constant/ConstantArrayTypeBuilder.php
@@ -269,7 +269,11 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 							continue;
 						}
 
-						$valueTypes[$i] = TypeCombinator::union($valueTypes[$i], $valueType);
+						if (!$optional && in_array($i, $this->optionalKeys, true)) {
+							$valueTypes[$i] = $valueType;
+						} else {
+							$valueTypes[$i] = TypeCombinator::union($valueTypes[$i], $valueType);
+						}
 						$offsetMatch = true;
 					}
 
diff --git a/tests/PHPStan/Analyser/nsrt/bug-11846.php b/tests/PHPStan/Analyser/nsrt/bug-11846.php
index 4f67410ad3d..b4a3bc4bb1d 100644
--- a/tests/PHPStan/Analyser/nsrt/bug-11846.php
+++ b/tests/PHPStan/Analyser/nsrt/bug-11846.php
@@ -38,16 +38,16 @@ function demo2(array $idList): void
 		$outerList[$id] = [];
 		array_push($outerList[$id], []);
 	}
-	assertType('non-empty-array{1?: array{}|array{array{}}, 2?: array{}|array{array{}}}', $outerList);
+	assertType('non-empty-array{1?: array{array{}}, 2?: array{array{}}}', $outerList);
 
 	foreach ($outerList as $key => $outerElement) {
 		$result = false;
 
-		assertType('array{}|array{array{}}', $outerElement);
+		assertType('array{array{}}', $outerElement);
 		foreach ($outerElement as $innerElement) {
 			$result = true;
 		}
-		assertType('bool', $result);
+		assertType('true', $result);
 
 	}
 }
diff --git a/tests/PHPStan/Analyser/nsrt/bug-14551.php b/tests/PHPStan/Analyser/nsrt/bug-14551.php
new file mode 100644
index 00000000000..9b31fd2729a
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-14551.php
@@ -0,0 +1,99 @@
+<?php declare(strict_types = 1);
+
+namespace Bug14551;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param non-empty-list<'a'|'b'> $keys
+ */
+function foreachTwoKeys(array $keys): void
+{
+	$result = [];
+	foreach ($keys as $k) {
+		$result[$k]['x'] = 1;
+		$result[$k]['y'] = 2;
+	}
+
+	assertType("array{a?: array{x: 1, y: 2}, b?: array{x: 1, y: 2}}", $result);
+}
+
+/**
+ * @param non-empty-list<'a'|'b'> $keys
+ */
+function foreachThreeKeys(array $keys): void
+{
+	$result = [];
+	foreach ($keys as $k) {
+		$result[$k]['x'] = 1;
+		$result[$k]['y'] = 2;
+		$result[$k]['z'] = 3;
+	}
+
+	assertType("array{a?: array{x: 1, y: 2, z: 3}, b?: array{x: 1, y: 2, z: 3}}", $result);
+}
+
+/**
+ * Test without foreach: union-key nested assignment
+ * @param 'a'|'b' $k
+ */
+function withoutForeach(string $k): void
+{
+	$result = [];
+	$result[$k]['x'] = 1;
+	$result[$k]['y'] = 2;
+
+	assertType("array{a?: array{x: 1, y: 2}, b?: array{x: 1, y: 2}}", $result);
+}
+
+/**
+ * Test with integer union keys
+ * @param 0|1 $k
+ */
+function integerKeys(int $k): void
+{
+	$result = [];
+	$result[$k]['x'] = 1;
+	$result[$k]['y'] = 2;
+
+	assertType("array{0?: array{x: 1, y: 2}, 1?: array{x: 1, y: 2}}", $result);
+}
+
+/**
+ * Test three-way union key
+ * @param 'a'|'b'|'c' $k
+ */
+function threeWayUnion(string $k): void
+{
+	$result = [];
+	$result[$k]['x'] = 1;
+	$result[$k]['y'] = 2;
+
+	assertType("array{a?: array{x: 1, y: 2}, b?: array{x: 1, y: 2}, c?: array{x: 1, y: 2}}", $result);
+}
+
+/**
+ * Required keys should still union (not replace) — existing behavior must be preserved
+ */
+function requiredKeysStillUnion(): void
+{
+	$result = ['a' => ['x' => 1], 'b' => ['x' => 1]];
+	/** @var 'a'|'b' $k */
+	$k = 'a';
+	$result[$k]['y'] = 2;
+
+	assertType("array{a: array{x: 1, y?: 2}, b: array{x: 1, y?: 2}}", $result);
+}
+
+/**
+ * Non-nested union-key overwrites with optional keys
+ * @param 'a'|'b' $k
+ */
+function nonNested(string $k): void
+{
+	$result = [];
+	$result[$k] = 1;
+	$result[$k] = 2;
+
+	assertType("non-empty-array{a?: 2, b?: 2}", $result);
+}