Thanks for your bug report. I have not been able to reproduce this issue. I tested this on a server by joining an empty channel, adding a nickname to my auto-op list, and then staring a new connection with that nickname and joining the same channel. If the first nickname on the channel has op, it sends a mode to op the newly joined nickname. If it doesn't have op, no mode is sent.
If you test this in a clean copy of mIRC, with no scripts, are you able to reproduce the issue? Can you provide a step by step method that reproduces this for you?