Respect binding priority.

src/Avalonia.Base/AvaloniaObject.cs

@@ -459,7 +459,7 @@ namespace Avalonia
             {
                 if (b.Instance(this, property, null) is not UntypedBindingExpressionBase expression)
                     throw new NotSupportedException("Binding returned unsupported IBindingExpression.");
-                return GetValueStore().AddBinding(property, expression);
+                return GetValueStore().AddBinding(property, expression, priority);
             }
             else
             {

src/Avalonia.Base/PropertyStore/ValueStore.cs

@@ -45,7 +45,8 @@ namespace Avalonia.PropertyStore
 
         public BindingExpressionBase AddBinding(
             AvaloniaProperty property,
-            UntypedBindingExpressionBase source)
+            UntypedBindingExpressionBase source,
+            BindingPriority? priorityOverride = null)
         {
             if (property.IsDirect)
             {
@@ -57,7 +58,7 @@ namespace Avalonia.PropertyStore
             }
             else
             {
-                var priority = source.Priority;
+                var priority = priorityOverride ?? source.Priority;
 
                 if (priority == BindingPriority.LocalValue)
                 {