I saw this question on Ask Ubuntu Meta "Firefox redirects askubuntu.com to google.com" which got migrated there from Ask Ubuntu by the community through the "belongs to another site" close votes.
However, I disagree and wanted to vote to migrate it back to main. This wasn't possible though, as the option in the close dialog was gone. So I raised a custom mod flag:
I believe this question belongs to the main site and the migration to Meta should be undone. It's a technical question about problems accessing a specific website, which theoretically could also been anything else but askubuntu.com.
It got marked as helpful with this comment, but no action was taken:
Perhaps, but we can't migrate it back. Apparently, it can only be done once.
So apparently even moderators can't revert a migration within the same site (main ←→ meta). Why is the system restricted this way? And what should be done in such a case instead?