Theory: the IP address seen by Postmill is that of a reverse proxy which sends the actual end user's IP thorugh X-Forwarded-For or similar. However, Postmill/the web server hasn't been configured to use these headers, and the random bans happen because the reverse proxy used in the moment of posting has an IP address with a ban meant for another user.

Also worth noting that users marked as 'trusted' aren't subject to IP ban checks, but explicitly marking everyone as trusted is a pain in the butt of course.