A 3D View is based on the layers contained within a 2D View Map frame. When two maps are overlaid, the original map frames cease to exist since the layers are merged into a new Map frame. Therefore, the original 3D View no longer has any data associated with it. You will need to regenerate a new 3D View from the overlaid maps if you wish to view them in 3D.
Updated September 25, 2017