diff --git a/Docs/ChangeLog-5x.md b/Docs/ChangeLog-5x.md
index eb2f5bf9..234512e8 100644
--- a/Docs/ChangeLog-5x.md
+++ b/Docs/ChangeLog-5x.md
@@ -17,6 +17,8 @@ The 5.5.0 release is a minor maintenance release.
   * **Update:** Update stb_image to v1.30.
   * **Update:** Update Wuffs to v0.3.4.
   * **Update:** Update TinyEXR to v1.0.13.
+  * **Bug fix:** Core library now correctly detects LDR error blocks when
+    compiled for AVX2 and returns max magenta rather than black.
   * **Bug fix:** Front-end wrapper now uses C++ RAII to manage lifetime of
     memory and file handles, fixing a number of resource leaks when on an error
     handling path.
diff --git a/Source/UnitTest/test_simd.cpp b/Source/UnitTest/test_simd.cpp
index 00595162..b72563d1 100644
--- a/Source/UnitTest/test_simd.cpp
+++ b/Source/UnitTest/test_simd.cpp
@@ -462,6 +462,30 @@ TEST(SuiteVfloat4, swz2)
 	EXPECT_EQ(r.lane<1>(), 2.0f);
 }
 
+/** @brief Test vfloat4 eq. */
+TEST(SuiteVfloat4, veq)
+{
+	// NaN should not be equal for anything
+	vfloat4 a(qnan, qnan, qnan, qnan);
+	EXPECT_FALSE(any(a == a));
+
+	// Non-NaN should be equal for everything
+	vfloat4 b(0.0f, 0.2f, -0.0f, -0.4f);
+	EXPECT_TRUE(all(b == b));
+}
+
+/** @brief Test vfloat4 neq. */
+TEST(SuiteVfloat4, vneq)
+{
+	// NaN should be unequal for everything
+	vfloat4 a(qnan, qnan, qnan, qnan);
+	EXPECT_TRUE(all(a != a));
+
+	// Non-NaN should not be unequal for anything
+	vfloat4 b(0.0f, 0.2f, -0.0f, -0.4f);
+	EXPECT_FALSE(any(b != b));
+}
+
 /** @brief Test vfloat4 add. */
 TEST(SuiteVfloat4, vadd)
 {
@@ -2166,6 +2190,30 @@ TEST(SuiteVfloat8, Loada)
 	EXPECT_EQ(ra[7], 7.0f);
 }
 
+/** @brief Test vfloat8 eq. */
+TEST(SuiteVfloat8, veq)
+{
+	// NaN should not be equal for anything
+	vfloat8 a = vfloat8_lit(qnan, qnan, qnan, qnan, qnan, qnan, qnan, qnan);
+	EXPECT_FALSE(any(a == a));
+
+	// Non-NaN should be equal for everything
+	vfloat8 b = vfloat8_lit(0.0f, 0.2f, -0.0f, -0.4f, 0.0f, 0.2f, -0.0f, -0.4f);
+	EXPECT_TRUE(all(b == b));
+}
+
+/** @brief Test vfloat8 neq. */
+TEST(SuiteVfloat8, vneq)
+{
+	// NaN should be unequal for everything
+	vfloat8 a = vfloat8_lit(qnan, qnan, qnan, qnan, qnan, qnan, qnan, qnan);
+	EXPECT_TRUE(all(a != a));
+
+	// Non-NaN should not be unequal for anything
+	vfloat8 b = vfloat8_lit(0.0f, 0.2f, -0.0f, -0.4f, 0.0f, 0.2f, -0.0f, -0.4f);
+	EXPECT_FALSE(any(b != b));
+}
+
 /** @brief Test vfloat8 add. */
 TEST(SuiteVfloat8, vadd)
 {
diff --git a/Source/astcenc_vecmathlib_avx2_8.h b/Source/astcenc_vecmathlib_avx2_8.h
index af4ecf78..8776b9cb 100644
--- a/Source/astcenc_vecmathlib_avx2_8.h
+++ b/Source/astcenc_vecmathlib_avx2_8.h
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // ----------------------------------------------------------------------------
-// Copyright 2019-2025 Arm Limited
+// Copyright 2019-2026 Arm Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
 // use this file except in compliance with the License. You may obtain a copy
@@ -640,6 +640,8 @@ ASTCENC_SIMD_INLINE vfloat8 operator/(float a, vfloat8 b)
 
 /**
  * @brief Overload: vector by vector equality.
+ *
+ * Returns vector of false mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask8 operator==(vfloat8 a, vfloat8 b)
 {
@@ -648,10 +650,12 @@ ASTCENC_SIMD_INLINE vmask8 operator==(vfloat8 a, vfloat8 b)
 
 /**
  * @brief Overload: vector by vector inequality.
+ *
+ * Returns vector of true mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask8 operator!=(vfloat8 a, vfloat8 b)
 {
-	return vmask8(_mm256_cmp_ps(a.m, b.m, _CMP_NEQ_OQ));
+	return vmask8(_mm256_cmp_ps(a.m, b.m, _CMP_NEQ_UQ));
 }
 
 /**
diff --git a/Source/astcenc_vecmathlib_neon_4.h b/Source/astcenc_vecmathlib_neon_4.h
index f31063d9..73d47dc9 100644
--- a/Source/astcenc_vecmathlib_neon_4.h
+++ b/Source/astcenc_vecmathlib_neon_4.h
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // ----------------------------------------------------------------------------
-// Copyright 2019-2024 Arm Limited
+// Copyright 2019-2026 Arm Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
 // use this file except in compliance with the License. You may obtain a copy
@@ -674,6 +674,8 @@ ASTCENC_SIMD_INLINE vfloat4 operator/(vfloat4 a, vfloat4 b)
 
 /**
  * @brief Overload: vector by vector equality.
+ *
+ * Returns vector of false mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask4 operator==(vfloat4 a, vfloat4 b)
 {
@@ -682,6 +684,8 @@ ASTCENC_SIMD_INLINE vmask4 operator==(vfloat4 a, vfloat4 b)
 
 /**
  * @brief Overload: vector by vector inequality.
+ *
+ * Returns vector of true mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask4 operator!=(vfloat4 a, vfloat4 b)
 {
diff --git a/Source/astcenc_vecmathlib_none_4.h b/Source/astcenc_vecmathlib_none_4.h
index 5c6a77eb..5e4157e0 100644
--- a/Source/astcenc_vecmathlib_none_4.h
+++ b/Source/astcenc_vecmathlib_none_4.h
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // ----------------------------------------------------------------------------
-// Copyright 2019-2025 Arm Limited
+// Copyright 2019-2026 Arm Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
 // use this file except in compliance with the License. You may obtain a copy
@@ -764,6 +764,8 @@ ASTCENC_SIMD_INLINE vfloat4 operator/(vfloat4 a, vfloat4 b)
 
 /**
  * @brief Overload: vector by vector equality.
+ *
+ * Returns vector of false mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask4 operator==(vfloat4 a, vfloat4 b)
 {
@@ -775,6 +777,8 @@ ASTCENC_SIMD_INLINE vmask4 operator==(vfloat4 a, vfloat4 b)
 
 /**
  * @brief Overload: vector by vector inequality.
+ *
+ * Returns vector of true mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask4 operator!=(vfloat4 a, vfloat4 b)
 {
diff --git a/Source/astcenc_vecmathlib_rvv_n.h b/Source/astcenc_vecmathlib_rvv_n.h
index bb032f24..8c47ef86 100644
--- a/Source/astcenc_vecmathlib_rvv_n.h
+++ b/Source/astcenc_vecmathlib_rvv_n.h
@@ -648,6 +648,8 @@ ASTCENC_SIMD_INLINE vfloat operator/(float a, vfloat b)
 
 /**
  * @brief Overload: vector by vector equality.
+ *
+ * Returns vector of false mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask operator==(vfloat a, vfloat b)
 {
@@ -656,6 +658,8 @@ ASTCENC_SIMD_INLINE vmask operator==(vfloat a, vfloat b)
 
 /**
  * @brief Overload: vector by vector inequality.
+ *
+ * Returns vector of true mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask operator!=(vfloat a, vfloat b)
 {
diff --git a/Source/astcenc_vecmathlib_sse_4.h b/Source/astcenc_vecmathlib_sse_4.h
index 7cf165b1..d91faa2e 100644
--- a/Source/astcenc_vecmathlib_sse_4.h
+++ b/Source/astcenc_vecmathlib_sse_4.h
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // ----------------------------------------------------------------------------
-// Copyright 2019-2024 Arm Limited
+// Copyright 2019-2026 Arm Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
 // use this file except in compliance with the License. You may obtain a copy
@@ -724,6 +724,8 @@ ASTCENC_SIMD_INLINE vfloat4 operator/(vfloat4 a, vfloat4 b)
 
 /**
  * @brief Overload: vector by vector equality.
+ *
+ * Returns vector of false mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask4 operator==(vfloat4 a, vfloat4 b)
 {
@@ -732,6 +734,8 @@ ASTCENC_SIMD_INLINE vmask4 operator==(vfloat4 a, vfloat4 b)
 
 /**
  * @brief Overload: vector by vector inequality.
+ *
+ * Returns vector of true mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask4 operator!=(vfloat4 a, vfloat4 b)
 {
diff --git a/Source/astcenc_vecmathlib_sve_8.h b/Source/astcenc_vecmathlib_sve_8.h
index 9f18853f..4408c81b 100644
--- a/Source/astcenc_vecmathlib_sve_8.h
+++ b/Source/astcenc_vecmathlib_sve_8.h
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // ----------------------------------------------------------------------------
-// Copyright 2019-2024 Arm Limited
+// Copyright 2019-2026 Arm Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
 // use this file except in compliance with the License. You may obtain a copy
@@ -615,6 +615,8 @@ ASTCENC_SIMD_INLINE vfloat8 operator/(float a, vfloat8 b)
 
 /**
  * @brief Overload: vector by vector equality.
+ *
+ * Returns vector of false mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask8 operator==(vfloat8 a, vfloat8 b)
 {
@@ -623,6 +625,8 @@ ASTCENC_SIMD_INLINE vmask8 operator==(vfloat8 a, vfloat8 b)
 
 /**
  * @brief Overload: vector by vector inequality.
+ *
+ * Returns vector of true mask values if a or b is NaN.
  */
 ASTCENC_SIMD_INLINE vmask8 operator!=(vfloat8 a, vfloat8 b)
 {