[MaDProFF]

Have you tried it on other slopes? instead of reversing in, drive in onto slop.

Does sound like a mechanical issue (not good)

Have they checked gearbox oil level

I am sure you may have tried putting handbrake on first, and then putting it in P, maybe the car will not roll to lock the gearbox.

Try every combination, so when you go to dealer you have a better understanding how to replicate. Even VIDEO problem sequence.

Nothing more annoying when you know you have a problem, but cannot explain or replicate to a dealer who thinks you are mad.