Can you measure the voltage output of the LDO using a multi-meter to see if it is actually out of range? If so, you might also want to measure the current draw; a voltage droop could be caused by the MCU drawing excessive current, which then causes a reset, which causes the MCU current draw to drop, which reverses the voltage drop allowing the MCU to power up, but then the MCU draws excessive current and everything repeats.
If the LDO output is actually with range, then perhaps try lifting the power good pin on the LDO and replacing it with a pull up. Then try to connect to the MCU at cold temp to see if it is still functional. You can also scope the lifted LDO pin to see if it still has repeat resets.
If the above rules out issues with the LDO power good signal and the issue seems to be with the MCU, the watchdog is the first place to investigate.