The only reasons I can come up with here are thus:
Could this be mIRC joining the path and then the file at the end of the path? Some sort of compensation for an accidental space between path and file.
/run a b c
will return:
ab c
In this instance, mIRC could be joining path and filename (a and b) and leaving a space for commandline switches (c) for operating that file.
However,
/run aa b
will work correctly and return aa b.
I can't explain this one tho
