moc registers the type with fully qualified name (which is ok) but the problem is when we use it in other places (e.g. a class property) it also must be fully qualified. e.g.:

namespace MyNS {
 Q_NAMESPACE
 enum MyEnum {
  Key1,
  Key2
 };
 Q_ENUM(MyEnum)

 class MyClass {
    Q_GADGET
    Q_PROPERTY(MyNS::MyEnum myEnum MEMBER myEnum)
    MyNS::MyEnum myEnum;
 }
}

As you can see MyClass is inside MyNS namespace but moc still needs to use MyNS::MyEnum instead of just MyEnum .