I had time to file a case - 01729857. Did a remote session today, I have an AP3825 and AP3710 in the same room, on the same channel - the AP3825 sees no traffic from the client device until the SSID is enabled on the AP3710, at which point the client associates fine to the AP3710. I also did a capture from a MacBook Pro, which can see probes from the client that don't appear in the RealCapture on the AP3825. Personally my money is on the SDK upgrade for AP38xx in 10.41 (since everything was fine with 10.31) but I guess we'll see what GTAC comes back with.
Also in another area the client bounces between 4 APs, switching every minute or so with the reason code for leaving just being 1, the ever informative "Unspecified reason".