It is possible that the AS/400 is indicating "search me" on it's CP Capabilities when it starts CP-CP sessions with SNASw. This means it will not register its LUs, but instead wants it's NN server to forward ALL Locate searches to it.
Take a look at the DLC trace next time the AS/400 connects in. In the CP Capabilities (x12C1), look at CV33, byte 2, bit 0. If this bit is on, the AS/400 requests to be searched.
I think this is pretty inefficient, as it takes away one of the features of SNASw, which is the ability to limit its role in network searches. I don't know how to control the "search me" feature in AS/400, but I recommend turning it off. This will cause the AS/400 to register its LUs, and receive searches only for LUs it actually owns.
This is the only explanation I can think of for what you are seeing.
Regards,
Bob