I ran into a similar bug in Git Lab where the v18 upgrade didn’t replace now-unsupported time zones so things like scheduled pipelines simply stopped running and were labeled inactive in the UI without explanation.
https://gitlab.com/gitlab-org/gitlab/-/issues/556779#note_26...
The fix is simply to change them to the non-backwards linked names but it caused some confusing API errors since data which would no longer pass validation was already in the database and didn’t look wrong.