Thanks for your bug report.
As maroon pointed out, the commands in the default popup menus in mIRC have been the same for a long time, probably about 20+ years, and have always worked without any issues. They are in almost every copy of mIRC that has ever been installed, so they are very much a core feature.
I can understand why the InspIRCd developers thought this would be a useful change, as it does provide feedback on excess parameters, but it breaks with how all other ircds have always worked, ie. excess parameters are ignored.
I can change the popup commands to use the format suggested by maroon in the next release but this issue would still be present in every existing installation of mIRC going back 20+ years for users on InspIRCd servers.