If it's not an issue with tire traction, my suspicion is that a small layer of ice is freezing on the pads, so they stop the car when they initially actuate, but after however many seconds the ice melts after absorbing enough heat from the rotors and the car rolls downhill.
I think someone should (safely!) try spraying the brake pads with a bit of water and then parking their car on an incline to see if they can replicate this problem.They may also need to brake enough to warm up the rotors, and will need to continue to spray the pads only until they see them ice up. Checking the rotor/pads with an IR thermometer after a drive should help with finding a good temperature/time range to try the experiment.
Another way to check could be to switch creep on. If you can't replicate the problem at all with creep on that suggests it's associated with one-pedal driving using the brakes a lot less in specific cold/wet environments.