oh ok, your acting as an intermediate layer, adjusting what are normal server messages, to meet your requiered wants
Nice!
Maybe its that the part/quits/kicks(?) are not being processed with an identicle filter? thus mirc sees them as different nicks parting?
If your acting as an intermediate layer you shouldnt need to do anything to the client level i would think, as that should be seperate alltogether from your filters.