“And it just so happens that all local accounts in a Mastodon instance have a null value in their URI field, so they all matched.”
One rename message from another Mastodon server kicked off an account merge. Replication lag ran the steps out of order, and the merge worker swept up every local account because they all had a null URI. 198 accounts were folded into a stranger on another instance before someone restarted the containers. A null that matches everything is a delete-all button with extra steps.