Thanks for your bug report. I have not been able to reproduce this issue here so far.

I installed a new copy of mIRC in a clean, empty folder, connected to a server, joined a channel, maximized it, and saved its position via the system menu. When I rejoin the channel later, and my windows are not maximized, the channel is maximized on join.

If you can provide a step by step method to reproduce this, please let us know.