The channel detaching feature is getting in the way
here. mIRC sends a part and join before the ircd
responds to the part. ZNC thinks that you are still on
the channel since the ircd hasn't notified you that you
have parted yet so it trys to reattach you instead of
passing the join along to the server.
Logged In: YES
user_id=1093455
This is now fixed and committed to cvs.