If I understand correctly, nextcloud automatically updated … which I didn’t think it would, normally. Maybe it’s a “feature” of the AIO docker image?

Never upgrade to the latest and greatest of … anything really, especially in production. Let others test it first, or as suggested already, have a staging environment where you test the upgrade first. I guess you can still downgrade nextcloud though, especially if you have a backup.

Are you using the AIO image? I don’t know how well that works, but yeah, I absolutely hate automatic updates like that. I tried it once and I decided to use the plain “official but not supported” docker image instead, where I manage things myself. Never had an issue, and I can control which version I’m running, I can backup to wherever I want, using whichever system I want, etc.