Apple TV+ - screen goes black when main feature starts

I know this will sound like I'm totally off my rocker, but this is really what's happening.

I can log into Apple TV+ with no problem. Screen looks normal, previews play as they should, with video and audio working just fine. However, when the main feature starts, the video goes black but the audio remains. This happens with everything I try to play. But! When I cast Apple TV+ content from my iPad/iPhone to my TV, the screen doesn't go black.

Can anyone shed any light on what might be going on, and how I can fix this? If neither the sound nor the video was working, this might make a bit of sense. But the fact that the previews play just fine, and then only the video kicks out…🤬 Any would be greatly appreciated!

Many thanks!