We recently upgraded to 09.21.12.0005 and now every AP shows only 1 Active Client in the AP Properties window on the controller. I'm running a pair of C5210 in HA with over a 1200 APs (mix of 3610, 3715 & 3825). I've checked the different AP models and it's the same across the board.
If I pull up an Client Report by AP on the controller, I see all the clients. Also, if I filter by AP name in OneView, I get all the clients. It's only on the AP properties page that it's not reporting correctly.