Examples such as $int(-2.4) proves mIRC incorrect. This should infact be -3.
According to your definition, yes. According to mIRC, no. There's no obligation for mIRC to conform to your definition, or any definition at all for that matter, so this is not a bug.
Anyway, instead of your scripted $_int you can just use $floor.
See also
this lengthy discussion about the issue.