Some rough estimates:
Even if it only affects 0.1% of users, that still puts the number of affected users in the thousands to 10s of thousands range, with an estimated 10-15 million Pixel 6-8 phones out there. T-Mobile (and Google Fi) haven't even allowed Android 14 to go out as well. So between the 800 stars and 500 comments on the bug, and the other 564 votes and 260 comments on the community forum post, and given that only a fraction of people do that, many are going to contact Google Support privately, or take the phone to BestBuy or wherever, that somewhat lines up.
I'd still think someone would've run into in the beta, particularly since beta users are even more tech savvy and likely to use multiple users. But perhaps not enough reports to get Google's attention.
But this is something that Google's automated testing should've caught. You don't have to do anything other than use the feature, and switch between accounts several times. Not a hard bug to hit at all.
Android's stock backup misses a lot of things, apps can opt-out and many do, it also doesn't backup files or documents or photos. Photos are "backed up" if you enable Google Photos cloud sync, but that's separate. It basically just backs up your phone calls/SMS, system settings and what apps you have installed (but not their data unless they tie into the backup APIs).