Unfortunately I am unable to reproduce this issue here. I have tested under Windows XP and 7 with both a LinkSys and a NetGear router and mIRC returns the correct results in all cases.

It is not clear whether the user reporting this issue is not able to get any UPnP features at all to work in mIRC or whether their issue is limited to /localinfo.

If you enable all of the UPnP options in the mIRC Options/Connect/Options/Port dialog and then enable Identd (turn off "enable only when connecting") and then use PortForward to check the UPnP settings on the router, does port 113 appear in the UPnP ports list? Is your External IP (as displayed by PortForward) correct?