Wanted to pop in with some of my findings on this poking around at youtube videos that worked and don’t work.
First off the issue appears to be down to this, here’s the video from the OP:
And here’s a friend’s video which does play:
This is consistent across basically all the ones I’ve checked, and would account for the codec changes mentioned before.
Now, the reason this is happening unfortunately seems to be down to views, it appears based on how many views a video has youtube will render a video in more codecs than just avc1 so I’m not really sure how to really fix this, it comes down to CEF or whatever is being used to play the videos simply not having support for the limited codecs, which as Mac has mentioned, he has no control over.
I did find this old reddit thread which delves in to an older variation of this issue with some solutions:
https://www.reddit.com/r/youtubegaming/comments/ju5am9/this_is_how_you_unlock_vp9_at_720p_and_1080p/
Unfortunately seems to come down to basically reuploading the video with different settings
that or getting more views on the video somehow.