This is not a bug.
You can still send the file using $shortfn($findfile(dir,*file*name*,1)) to send it in 8.3 format. However, no mIRC command will allow multiple internal, leading or trailing spaces. Therefore, it's best simply to rename the file.
//var %filename = $replace($findfile(dir,*file*name*,1),$chr(32),>) | rename $shortfn($replace(%filename,>,$chr(32)) $+(",$nofile(%filename),$gettok($nopath(%filename),1-,32),")
and then send it normally.