Firstly, apologizes that you are having trouble with the extension. Can you tell me a bit more about the project itself? Perhaps there is a use case I missed for detecting view size.
What version of GM are you using? Targeted platform? Are you using the application surface or no? What are your view sizes in each room? Viewport size? GUI size?
As a temporary fix until the issue is figured out you can try using the force surface size hack. In the create event of ubg_transition_obj change the variable hack_force_surface_size to match that of your view's render resolution.
All the best.