Server Chunk Cache Distribution

Its a weird one:

The server is running behind a NAT. The NAT has port forwarding to:

192.168.0.15:25565 and
192.168.0.25:25566

When attempting to connect on port 25565 (by router or in network) connection succeeds an operates normally.
When attempting to connect to port 192.168.0.25:25566 from in network connection succeeds and operates normally.
When attempting to connect to port 25566 from out of network connection succeeds, HOWEVER, the multi-player chunk cache is never populated.

What's the difference between the two IPs? IP .15 is operating over a wireless channel provided by a USB WiFi antenna (ASUS model N13), the IP .25 is operating over a CAT 5e ethernet cable. Both operate on the same computer. Both server instances have appropriately configured properties (as evidence by successful connection)

In all other respects these connections to the router are identical.