[fixed] Upgrade 28.0.0 stable ... is NOT stable - was broken index data from old "Social APP"

Not a good idea imho.

I’m confused. Are you still stuck in maintainance mode on that instance or not? Because earlier in your post you wrote the following:

occ db:add-missing-indices
occ maintenance:repair
occ app:update --all
or
occ upgrade

fixes this issue

Looks like you’re not the only one experiencing this issue:

https://help.nextcloud.com/t/version-28-0-0-11-safari-issues/176528

I guess someone should file a bug report on GitHub, if no one has already done so.