Hi Andy,
Thank you for sharing this info. The firmware upgrade should have happened at the same time, provided the AP Zone firmware was changed at the same time of the upgrade for these APs. Regarding the 6 AP's reboot because of a system fault, this does not sound normal to me. I would request you to log a support case along with the supportinfo files of these 6 AP's for us to investigate.
Regards,
-Ankush