It seems mIRC doesn't support/honour the MODES capability setting as defined by server numeric response 005. Example:

ircd-ratbox-2.2.8(20080111_7-24916). irc.server.com egGHIKMpZ TS6ow 1SC
CHANTYPES=&# EXCEPTS INVEX CHANMODES=eIb,k,l,imnpst CHANLIMIT=&#:16 PREFIX=(ov)@+ MAXLIST=beI:75 NETWORK=mynet MODES=4 STATUSMSG=@+ KNOCK CALLERID=g are supported by this server

While selecting 4 nicknames in the GUI and choosing Control -> Op results in:

[05:53] * mynick sets mode: +ooo nick1 nick2 nick3

Reference material:

http://www.irc.org/tech_docs/005.html
http://www.irc.org/tech_docs/draft-brocklesby-irc-isupport-03.txt