Symptom:
Phone models affected 
6921
6941
6945
Firmwares tested: 9-3-3-2, 9-3-3-2SR1, 9.2.4, 9.2.2, 9.4.1
Protocol:SIP/SCCP

Other phone models not impacted

Conditions:
The phone reset happens every 12 and half or 25 hours. And the pattern is continuous. If a phone resets at time X. Then next it will reset at X+12.5 or X+25 hours correctly and will register back. Where a2 or 25 is the value of the DHCP lease time which has been currently set.

The reason device has unregistered will be  with Reason=8 which is the following 
DeviceInitiatedReset - The device has initiated a reset, possibly due to a power cycle or internal error. No action required; the device will re-register automatically. As seen the devices register back on the same second itself.

From the phone captures while the phone is restarting you can see that it is renewing its DHCP-lease and then restarts.The issue is related to the DHCP-lease renew procedure.