We encountered the same issue. It only happens with UserWidget in a slot. Widget in a slot is safe. As a workaround you can wrap all UserWidget in a slot with a UBorder (Widget). That saves it.
It also only happens if the Parent Widget is not referenced by instance in another widget. So you can fix it by placing your Parent Widget in a HUD or something. It only happens when your Parent Widget is fresh referenced via TSubclassOf. Reference with SoftPtr might also fix it.