I suspect this is due to a bug in auto-content removal. The ibbit.at communities only have posts created by bots that are mirroring an rss feed into a community. piefed.social is configured to automatically prune bot-created posts that don't have engagement (comments iirc) after a set amount of time. So, I am willing to bet that somewhere in that removal process, the removal is being counted as more than -1 post, driving the total for the community below zero.