I can replicate in the latest beta, 7.61.961. Which linux distro are you using? I can replicate Ubuntu (64-bit), Debian (64 bit) , Alpine Linux (32-bit).
I guess I should be asking, is mIRC doing something weird, undocumented, or? How does it work? Might be a bug in WINE?
mIRC is only compatible with Windows XP/Vista/7/8/10