- Power off your phone
- Press and hold vol up and power key until you are in recovery mode
- use vol up and down to navigate
- select Wipe Cache Partition
- press power key to confirm
- click yes
- click reboot system
It seems unlikely, especially on both devices, but did you enable Auto Restart by any chance?
In reality, almost none of issues reported by people, have anything to do with any updates.
As @Tallman suggested, wiping Cache after any Security Patch Update is a good idea, I always do, and I've never had any issues with any of my Samsung devices.
If you have identical apps on both, it's possible that there's an incompatible app causing the Reboot as well.
You could try Booting into Safe Mode & see if the behaviour continues?
You mean that you used Odin to Flash the previous version of the Firmware?
Since you already Wiped Cache, I'd probably try Booting into Safe Mode as I suggested previously.
The ONLY time that I've ever had random Reboots was while testing Custom Roms (part of the territory) and that's usually caused by a wonky Kernel, but I've never had it happen with Samsung's stock Firmware.
I would suspect that it's a 3rd party app causing the issue, why is why I was asking if you had the same apps installed on both devices.
Anyway, try Safe Mode for the amount of time that your devices would have normally Rebooted on their own.
Are they plugged into a charger at the time by any chance?