On my ATV 4K I get that a lot with apps like CuriousityStream too. The video will start out in what looks like old SD TV resolution (480i) with all kinds of artifacts on the screen. Basically it looks like OTA antenna reception that is really poor. But after a minute are so it clears up and looks like decent 4K. I think it is likely the fault of the streaming service. Ever since tvOS 15 my Philo app has been crashing a lot. Philo rushed out an update to deal with the issue, but it didn’t resolve it. So I’m having to view Philo on my Fire TV Cube, which I also use for Amazon Prime. BTW, Prime looks okay on my Fire TV Cube, so it is probably just an issue with the Prime app on tvOS.