I can confirm my project(GMC Jam 40 entry Escape of the Clowns) was also having this issue. I'm not sure if it has to do with the fact that my room is 1920x1080, which is the same size as the monitor, and so GMS is slightly scaling the thing down in windowed mode. It was also doing it in full-screen IIRC.
The same fix you mentioned ALSO worked perfectly for mine. Since you want info, I'm not using any views, cameras, anything like that in this project. All the action in every single room is the same 1920x1080 room size, Viewports are not enabled in the room editor anywhere either.