I hadn’t seen this frontend before. Its nice. Yeh quality selection does seem a bit buggy for me also (librefox). I see under the user settings (click account top right), there are some settings for default quality and mpeg-dash. I havent tested myself but just spotted them. Maybe try these ?
Edit: i skipped your mention of “installed “ viewtube. We may not even be looking at the same thing lol. Im looking at viewtube.io . Perhaps the same if youre selfhosting anyway.