I've had a few issues with that recently. For the most part, mine remember the sorting order (A-Z for me too), but I know they reset after a software update on 2/8. The other morning I woke up and my two HR24s had reset their order, but they were also in some weird state (partial blue lights on front, not all of them), and there had been no software upgrade.
I'm pretty ambivalent about it though, yellow button and two clicks gets it back. This type of thing usually really bothers me, but I'm a recent switcher (less than a month) from Comcast, so I'm still in my honeymoon phase.