I think they are clamping down and trying to force people to watch yt in Google's desired way. They have managed to stop most Piped instances via ip banning and also requesting users to "sign in to confirm you are not a robot". I wish their was a proper alternative at this point. It would be great if something like Peertube took off.
Whenever I have issues with YouTube refusing to do things it used to do, I stop using it for a while and eventually they put it back. If you're not willing to do that, I find that the NewPipe app is better than the native YouTube app. But be warned that occasionally Google makes changes that break New Pipe and you need to wait a couple days for the devs to catch up to the change.
I've gone out of my way to make a cracked .IPA of YouTube (basic adocker + sponserblock + Downloader) for my phone to not have to deal with there stuff and it's held out so far. Just hope it keeps doing so
It could be that it's playing in a codec that your mobile browser doesn't support like av1 or something. Not sure though, is this happening with every video?
I tried last night to view a video I saw someone on SpaceHey had in their main page and couldn't do it on my phone or desktop browser. Phone wasn't in any way doing anything outside of the standard ublock origin on Mull, which has never had issues if I ever needed an "emergency" yt lookup. Desktop browser Firefox based with libredirect wasn't working since I've been having issues connecting and using Invidious and Piped instances for a while.
Ended up having to switch to Freetube on my desktop to view the video and it was worth it. The errors I kept getting where to sign in to confirm I'm not a bot.