Adding a KA75330ZTA Voltage Detector across power and reset appears to have stopped all issues caused by random power cycling on my custom PCB with RAK3172 running RUI3 4.0.1.
Radio still works and have not been able to brick RAK3172 after many, many random power cycles. I have run random power cycles for an hour and all is still well!!!
I feel I can now use RAK3172 in production.