If it's of any use, I'm including a log of reboots and upgrades from a single AC2 that's been running for about 10 months where I have with the NTP package installed, IPSec enabled (but very rarely used), and watchdog enabled. Although there have been many config changes over time, the ones I just mentioned have been static since the beginning.
Installed Apr/12/1019
Now Running 6.44.2
apr/26/2019 14:19:12 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.44.3
jul/01/2019 16:42:29 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.45.1
jul/22/2019 19:35:44 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.45.2
aug/05/2019 18:21:46 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.45.3
aug/18/2019 06:22:49 system,error,critical router was rebooted without proper shutdown
sep/06/2019 21:47:37 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.45.5
oct/10/2019 22:38:26 system,error,critical router was rebooted without proper shutdown
oct/12/2019 16:48:37 system,error,critical router was rebooted without proper shutdown
oct/26/2019 04:28:40 system,error,critical router was rebooted without proper shutdown
oct/27/2019 04:27:37 system,error,critical router was rebooted without proper shutdown
oct/28/2019 20:06:15 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.45.7
nov/18/2019 01:00:03 system,error,critical router was rebooted without proper shutdown
nov/18/2019 07:37:49 system,error,critical router was rebooted without proper shutdown
nov/29/2019 01:09:46 system,error,critical router was rebooted without proper shutdown
dec/03/2019 18:09:15 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.46
dec/08/2019 10:19:04 system,error,critical router was rebooted without proper shutdown
dec/09/2019 19:14:04 system,error,critical router was rebooted without proper shutdown
dec/12/2019 00:22:00 system,error,critical router was rebooted without proper shutdown
dec/12/2019 06:37:05 system,error,critical router was rebooted without proper shutdown
dec/14/2019 00:12:33 system,error,critical router was rebooted without proper shutdown
dec/14/2019 05:04:54 system,error,critical router was rebooted without proper shutdown
dec/17/2019 16:58:58 system,error,critical router was rebooted without proper shutdown
dec/20/2019 23:28:53 system,error,critical router was rebooted without proper shutdown
jan/05/2020 09:27:50 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.46.1
jan/06/2020 15:26:26 system,error,critical router was rebooted without proper shutdown
jan/07/2020 22:27:58 system,error,critical router was rebooted without proper shutdown
jan/17/2020 20:37:36 system,error,critical router was rebooted without proper shutdown
jan/18/2020 16:15:49 system,error,critical router was rebooted without proper shutdown
jan/20/2020 14:32:13 system,error,critical router was rebooted without proper shutdown
jan/21/2020 13:30:42 system,error,critical router was rebooted without proper shutdown
jan/25/2020 22:26:18 system,error,critical router was rebooted without proper shutdown
jan/30/2020 22:49:14 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.46.2
jan/31/2020 07:46:11 system,error,critical router was rebooted without proper shutdown
feb/03/2020 15:58:29 system,error,critical router was rebooted without proper shutdown
feb/04/2020 16:25:24 system,error,critical router was rebooted without proper shutdown
feb/06/2020 17:59:23 system,error,critical router was rebooted without proper shutdown
feb/07/2020 13:17:24 system,error,critical router was rebooted without proper shutdown
feb/07/2020 16:13:36 system,error,critical router was rebooted without proper shutdown
feb/10/2020 07:58:24 system,error,critical router was rebooted without proper shutdown
feb/11/2020 14:42:14 system,error,critical router was rebooted without proper shutdown
feb/11/2020 20:21:46 system,info,critical Firmware upgraded successfully, please reboot for changes to take effect!
Now Running 6.46.3 and NTP Package Disabled
For this device, the reboots started somewhere around the 6.45.3 - 6.45.5 timeframe. I disabled the NTP package (still have the NTP client running) about 3 days ago, and so far no reboots. (I also upgraded to a newer firmware... perhaps not the smartest move to make two changes at once I admit.)
If I do see a reboot, I'll be sure to report back.