Unable to view movies without moving mouse constantly

First time attempting to watch a purchased and downloaded movie with macOS 10.14.5 and iTunes 12.9.5.5. Never had a problem prior to this with previous versions.

Now, no matter which movie I try to watch, if I don't constantly keep the Magic Mouse 2 moving around, there's sound, but only a black screen. Move the mouse, video instantly appears. Stop moving the mouse, black screen.

Any ideas? Thanks!

Edit: Seems it only happens in Full Screen view. When it goes to hide the on-screen controls, it also hides the picture. Works fine in windowed-mode. Annoying, to say the least!