It's not the ircd that decides how to display nicks locally in mIRC
mIRC displays: [timestamp] "<" [prefix] <nickname> ">" <message>
If there is a prefix, mIRC only chooses the greatest user mode, not all.
For example, mIRC will always display: <@nick> message, regardless of how many modes the ircd reports (and assuming @ is the greatest of said modes)
However, If you're using a custom script that does theming, it may use the code:
echo -t # $+(<,$nick(#,$nick).pnick,>) $1-
In such a case, mIRC will display all modes, since $nick(#,$nick).pnick returns all of them. This might have been the issue you were seeing. But again, the ircd controls none of this.