That's an interesting effect. I've always seen the opposite. If I do not specify the exact IP allocated to the NIC I want it to listen on, the port is not open, and no communication occurs.
I'll have to check with different server releases, but... yea, I actually advise people to check the IP and port the server is binding to with PortMon in one of my guides, and always do that as a matter of course when setting up a new server. So I know it's never happened to me before. (not while I was watching)
My firewall would block communication on any IP I hadn't designated though, so it's not something I worry about as much as I try to keep any and all such resource consumption to a minimum and lower potential attack footprint. As I'm sure you are doing.
See if you can spot anything odd in the WinSock APIs in your server, they should specify the exact IP you stated in the config (hotuk.ini, usually) during bind. If they are, check your system libraries, because PT uses some massively old headers, and the wrappers to the modern interface could be over-compensating in the name of "compatibility". (Microsoft have a tendency to do that)