This method has never worked and it’s not supported. You can’t make a UserWidget with a designed UI, then attempt to derive from it again. It’s unclear what the expectation would be. You can make a UUserWidget in C++ without a widget tree, then reparent a Widget Blueprint to the user derivative of UUserWidget in the editor.
Additionally, constructing a Blueprint from the perspective of the editor is different from creating a Widget Blueprint, they’re entirely different classes.