Apparently one of the lemmy.ml admins was overzealous in banning all User-Agent strings that contained the string “bot”. Bans were entered for all of the individual strings containing that word which were observed in their webserver logs, which impacted kbin’s reported agent of “kbinBot”.
The issue has been fixed, and I observed that one of my kbin posts to a lemmy.ml community was successfully pushed to the original instance.
Like I said, a blind sort by volume of the top n user agents in their logs containing the word bot would be enough to do it. Drop the output of that sort into a text file or a hash table, then create a user agent filter in the nginx config that blocks the specific strings seen in that file.
It is very much the sort of thing that a single admin can do by accident, and the exact sort of problem I would expect to see with rapidly growing instances operated by a very small number of tech enthusiasts.