This thread is a bit older, but here is the answer I just stumbled uppon when looking up the same problem.
regards
Edit: After one day of work with the solution in the link, I came to a new problem using this methode. It does work, at least for what it is supposed to do, update stuff within the editor. But OnSynchronizeProperties() (see link) is also called at least once when the widget is created.This will call what ever it should Syncronize, e.g. the text of a TextBox. But the TextBox->SetText() function call will break the binding on the TextBox.Text. This is also stated in the documentation: Drive UI Updates with Events | Unreal Engine Documentation (at the bottom). Other property bindings keep working. A quick fix would be adding OnTick() to the parent class and call OnSynchronizeProperty() within the tick and thus would emulate the binding to a variable. This is to often? Bindings get called once or twice a tick anyway, see: Question on UMG binding's update - Blueprint Visual Scripting - Unreal Engine Forums