I upgraded from 7.73 to 7.74.
It wasn't happening in 7.73.
I mostly use Slashnet, but just to try it out I just also connected to libera.chat and there the same thing happens.
By the way, while my post was pending here I started doubting whether this should be fixed in mIRC or in Wine, so I also made a bug report in the Wine project:
https://bugs.winehq.org/show_bug.cgi?id=55406
