https://qt-project.atlassian.net/browse/QTBUG-149607
https://bugs.kde.org/show_bug.cgi?id=508377
https://mail.kde.org/pipermail/distributions/2026-September/001731.html
--- a/src/qml/qml/qqmlglobal.cpp
+++ b/src/qml/qml/qqmlglobal.cpp
@@ -1113,7 +1113,7 @@
     // A non-composite type will always have a metaobject.
     const QMetaObject *typeMetaObject = type.metaObject();
-    const QQmlPropertyCache::ConstPtr typePropertyCache = typeMetaObject
-            ? QQmlPropertyCache::ConstPtr()
-            : QQmlMetaType::findPropertyCacheInCompositeTypes(type.typeId());
+    QVarLengthArray<QQmlPropertyCache::ConstPtr, 4> allPropertyCacheCandidates =
+            typeMetaObject ? QVarLengthArray<QQmlPropertyCache::ConstPtr, 4>{}
+                           : QQmlMetaType::rawCompositePropertyCachesForType(type.typeId());
 
     if (const QQmlData *ddata = ddata_for_cast(object)) {
@@ -1135,5 +1135,5 @@
                 // Multiple property caches can hold the same metaobject, for example for
                 // versions of non-composite types.
-                if (propertyCache == typePropertyCache.data())
+                if (allPropertyCacheCandidates.contains(propertyCache))
                     return true;
             }
@@ -1141,9 +1141,9 @@
     }
 
-    // If nothing else works, we have to create the metaobjects.
+    // If nothing else works, we have to create the metaobjects (if we can).
+    if (!typeMetaObject && !allPropertyCacheCandidates.isEmpty())
+        typeMetaObject = allPropertyCacheCandidates.first()->createMetaObject();
 
-    return object->metaObject()->inherits(typeMetaObject
-            ? typeMetaObject
-            : (typePropertyCache ? typePropertyCache->createMetaObject() : nullptr));
+    return object->metaObject()->inherits(typeMetaObject);
 }
 
--- a/src/qml/qml/qqmlmetatype.cpp
+++ b/src/qml/qml/qqmlmetatype.cpp
@@ -1491,4 +1491,18 @@
  * \internal
  *
+ * Returns all candidate property caches for a composite
+ * metatype instead of only the last inserted one.
+ * compare rawPropertyCacheForType (which handles however also non-composites)
+ */
+QVarLengthArray<QQmlPropertyCache::ConstPtr, 4>
+QQmlMetaType::rawCompositePropertyCachesForType(QMetaType metaType)
+{
+    const QQmlMetaTypeDataPtr data;
+    return data->findPropertyCachesInCompositeTypes(metaType);
+}
+
+/*!
+ * \internal
+ *
  * Look up by QQmlType and version. We only fall back to lookup by metaobject if the type
  * has no revisiononed attributes here. Unspecified versions are interpreted as "any".
--- a/src/qml/qml/qqmlmetatype_p.h
+++ b/src/qml/qml/qqmlmetatype_p.h
@@ -24,4 +24,5 @@
 
 #include <QtCore/qtyperevision.h>
+#include <QtCore/qvarlengtharray.h>
 
 QT_BEGIN_NAMESPACE
@@ -172,4 +173,8 @@
             QMetaType metaType, QTypeRevision version);
 
+    // All property caches for a composite metatype, which may map to more than one of them.
+    static QVarLengthArray<QQmlPropertyCache::ConstPtr, 4> rawCompositePropertyCachesForType(
+            QMetaType metaType);
+
     static bool canConvert(QObject *o, QMetaType metaType);
     static bool canConvert(const QQmlPropertyCache::ConstPtr &from, QMetaType metaType);
--- a/src/qml/qml/qqmlmetatypedata.cpp
+++ b/src/qml/qml/qqmlmetatypedata.cpp
@@ -258,4 +258,16 @@
 }
 
+QVarLengthArray<QQmlPropertyCache::ConstPtr, 4>
+QQmlMetaTypeData::findPropertyCachesInCompositeTypes(QMetaType t) const
+{
+    QVarLengthArray<QQmlPropertyCache::ConstPtr, 4> result;
+    const auto [begin, end] = compositeTypes.equal_range(t.iface());
+    for (auto iter = begin; iter != end; ++iter) {
+        if (auto cache = propertyCacheForPotentialInlineComponentType(t, iter))
+            result.append(std::move(cache));
+    }
+    return result;
+}
+
 void QQmlMetaTypeData::clearCompositeTypes()
 {
--- a/src/qml/qml/qqmlmetatypedata_p.h
+++ b/src/qml/qml/qqmlmetatypedata_p.h
@@ -24,4 +24,5 @@
 
 #include <QtCore/qset.h>
+#include <QtCore/qvarlengtharray.h>
 #include <QtCore/qvector.h>
 
@@ -120,4 +121,8 @@
     QQmlPropertyCache::ConstPtr findPropertyCacheInCompositeTypes(QMetaType t) const;
 
+    // Same, but returns all matches rather than only the last inserted one.
+    QVarLengthArray<QQmlPropertyCache::ConstPtr, 4> findPropertyCachesInCompositeTypes(
+            QMetaType t) const;
+
     static QQmlPropertyCache::ConstPtr propertyCacheForPotentialInlineComponentType(
             QMetaType t, const QQmlMetaTypeData::CompositeTypes::const_iterator &iter);
--- a/src/qml/qml/qqmlpropertyvalidator.cpp
+++ b/src/qml/qml/qqmlpropertyvalidator.cpp
@@ -3,4 +3,5 @@
 // Qt-Security score:significant
 
+#include "qqmlmetatype_p.h"
 #include "qqmlpropertyvalidator_p.h"
 
@@ -782,8 +783,27 @@
             // Determine isAssignable value
             bool isAssignable = false;
-            QQmlPropertyCache::ConstPtr c = propertyCaches.at(binding->value.objectIndex);
-            while (c && !isAssignable) {
-                isAssignable |= c == propertyMetaObject;
-                c = c->parent();
+            QQmlPropertyCache::ConstPtr source = propertyCaches.at(binding->value.objectIndex);
+
+            const auto inheritsFrom = [&](const QQmlPropertyCache::ConstPtr &target) {
+                for (QQmlPropertyCache::ConstPtr c = source; c; c = c->parent()) {
+                    if (c == target)
+                        return true;
+                }
+                return false;
+            };
+
+            isAssignable = inheritsFrom(propertyMetaObject);
+
+            if (!isAssignable) {
+                // A single (composite) metatype can map to multiple property caches when
+                // there are multiple engines; rawPropertyCacheForType only returns one of
+                // them. For non-composite types this yields an empty list and is a no-op.
+                const auto candidates = QQmlMetaType::rawCompositePropertyCachesForType(propType);
+                for (const auto &candidate : candidates) {
+                    if (inheritsFrom(candidate)) {
+                        isAssignable = true;
+                        break;
+                    }
+                }
             }