Thanks for your bug report. I have not been able to reproduce this issue. Here are the steps I am taking to test this:
In status window #1:
1) Connect to Efnet
2) Join #channel
3) Set +k password on #channel
In status window #2
1) Connect to Efnet
2) Join #channel
3) mIRC uses the correct password to join #channel
If you follow the above steps exactly, can you reproduce the issue?