I tested again under Windows 7/8 and XP SP3. I looked into the CStaticPortMappingCollection::Add() call in the code and mIRC v7.32 is passing 192.168.x.x to that routine under all three Windows versions. So the cause of the issue in your situation seems to be the routine that retrieves the interface IP address, which is being returned as 127.0.0.1 for you, however as far as I can tell it is working correctly here.
If you are using Windows XP SP2, that may be the cause of the issue, as SP3 included a wide range of networking fixes. Unfortunately I no longer have XP SP2 installed, so I cannot test this.
