https://git.pleroma.social/pleroma/pleroma/-/blob/develop/lib/pleroma/web/activity_pub/activity_pub.ex?ref_type=heads#L1623
Username generator uses the host value of URI.parse which returns raw IP without the brackets of port. I guess it could be fixed by simply replacing the value with authority (which returns both the bracketed address and port if present), but I'm not sure if this won't break anything else.