Here is my take on this, I believe two things could be happening, potentially even both, but neither one seems to be an "issue" just requires a bit of a learning curve.
1- I get in the car, I put my foot on the brake and press the ignition button, I'm not looking at the screen to see the E-Tron Ready prompt and I just go straight to D. It's easy to do this, because there is no engine turning on for the driver to know the car is ready for being put into drive.
Solution: Enter car, foot on brake, press ignition while watching the screen and wait for the e-tron ready sign to clearly be on the display before moving from P to D.
2- Although I'm nearly sure the issue is the one above for me, another problem could be, when I first put my foot on the brake, it's super stiff at a very high point in the travel, as soon as I press the ignition button, the brake mechanism mushes quite a bit and if I don't have enough pressure on the pedal to ensure the car registers a safe "P to D" timing. Again, I think the main issue is #1 above and in the past two to three weeks, I followed my instructions above as the solution and the issue has not occurred again.
The fact that the car rolls, is likely somehow related to how things work between the transmission, electric motor and gas motor, but I'm not knowledgable enough, I am thinking this is a non-issue as well as long as you follow the steps above to prevent the premature move from P to D.
Yes, once this mistake happens, you must return to P, turn off ignition, put foot on pedal, turn on ignition and wait for etron ready, then move from P to D and up up and away you go!
I hope this is helpful, I'm pretty certain about my findings as I was running into the issue you report several times a week as a brand new owner (never had a PHEV before) and now, it hasn't happened in two weeks, except for this morning where I again, by accident because i was in conversation, moved from P to D before etron was ready. To me, this is a learning curve issue because we no longer have the starting noise of the engine to know when the shifter is ready for P to D.