New update: after 5 days, the problem has manifested again.
If I'm on a Mac connected to the MAP and run Airport Utility, it shows all Apple Access points as "can't find them", even if one of them is connected to the same MAP as my Mac.
Rebooted the MAP, forcing all wireless clients to move to the RAP - boom, everything is suddenly working. 😞
So the mystery persists - things work for a while, then stop. Actually this looks similar to the problem I had when I was running only a single R710 in a standalone mode (non-Uleashed). Had to reboot it once a week to restore the Apple services. 😞