Xbox One S doesn't always show 4K?

There are videos in the YouTube app on my Xbox One S that I know are in 4k UHD (because I can play them in 4K UHD on my TV using the YouTube app), but I still only select 1080p can. But this only affects some videos, because I can play others in 4K UHD. I have set the setting on my Xbox to 4K UHD and my TV also shows that the Xbox is outputting a 4K signal.

Maybe these videos are only uploaded in 1080p.

No, since they run in 4K in the app on my TV

Start anew

Has this only happened recently? Currently there are throttling on YouTube, then 4K videos are only available in FullHD.

I thought that too, but I can watch other videos in 4K and in the app on my TV the videos also run in 4K. If you screwed the quality down then everyone would. Or can this have something to do with the coding in which they were uploaded?

Well, the quality on the devices is not necessarily correct, it can also be simple that your XBox is simply throttled very strongly by the router, but I think this is very unlikely, since you can't even select 4K → locked by the app. (see Amazon Prime Video there's just no 4K on the devices due to copy protection)

I think I have a solution for you, try your XBox in your video output settings to 4K UHD (you already have it) AND color depth 30 (10-bit) or 36 bits per pixel (12-bit).

It could be that the YouTube app limits the playback on the Xbox because it assumes a TV as output, that the playback is limited to 1080p if the output is not as HDR (10-bit - 12-bit, most TVs support the, PC- Monitors usually only the expensive) is recognized.