Oh, I see what you mean now..

I don't think that it is due to anything that mIRC has any control over. mIRC doesn't tell the server what your IP and hostname are. The IP is known from the socket connection, and the hostmask is looked up through the server's DNS. Maybe ask the IRCOps on those networks about what could be causing that problem. Or maybe ask other users if they are having the same issue.

-genius_at_work