I still think the problem may actually be with the saving step. Have you tried to add a "Save" button (just for testing), instead of automatically saving after the video has finished? I would also try to test without the MobileAdvert plugin. Also, add "No save" behavior to everything you don't need to save.
I'm out of ideas. If you manage to reproduce this problem in a small demo project, you can submit it as a bug:
github.com/Scirra/Construct-3-bugs/issues