Apple TV Flaky on Fire TV Stick 4K

I have a Fire TV Stick 4K device (running FireOS 6.2.8.1) and have loaded Apple TV+ (version 6.1) onto it.

Sometimes the AppleTV+ app works, but a lot of the time while I can get to the main screen, and view things in my library and so on, when I try to watch something from Apple TV+ I get some variant of the message

Video Unavailable

There is a problem loading this video

There is no issue with the internet connection - the same device streams from Netflix / Amazon / NowTV / BBC / etc with no problems, it is just Apple TV + that shows this issue.

Also, when this happens if I open a laptop on the same network and run Apple TV on that, I can open / view the content without problems.

All of which suggests that the issue is with the Apple TV+ app, not the hardware or the internet connection.

I have found that restarting the Fire TV Stick 4K device sometimes helps, but it is not guaranteed to work, other times simply leaving and returning some hours later and I can watch something. More than once I have paused watching something to do something else and upon returning the programme will not resume.

I have a 100Mbps download internet connection, and as noted other streaming apps on the same device work without issue.

Does anyone have any guidance on how I can resolve this issue?