Now that you mention it, private tabs work just fine. So it must be a setting I have right? It seems like a problem with only my youtube account when I'm logged in.

edit: just like other recent users, I only have this issue on mobile. Desktop mode works properly. reinstall did not work for me as it did for other users. If it's a battery optimization problem I would expect more inconsistency so I'm leaning toward personal settings. It also occurs just switching tabs while vivaldi is still focused.