This would be a more interesting question if God forbid somebody failed their Step 2. (although even then, I'd think the answer would simply be to retake Step 2 and sweep the whole sordid affair under the rug)
1. The Match is over (thus both updating of ERAS - including scores is also over)
2. You've already matched - why would your PD call to b^&ch you out? There is nothing he/she can do about it as you matching there is a contract that neither of you can back out of! It wouldn't make any sense.
3. Your PD will be MUCH more concerned that you pass the Derm in-service exams and eventually boards.
To extrapolate another step, what about failing the Step 2 CS. I know a lot of people who have only recently taken it and not found out their scores (myself included). Although I feel confident that I passed, there are a buncha stupid reasons to fail the CS (forget to wash your hands, forget to re-tie a patient's gown, etc).
So, considering that rescheduling the Step 2 CS requires about 6 months of time, what would happen then?
regarding the failing step 2 cs question, the bigger problem is that you often can't graduate med school without having passed it (varies by school). your derm program probably won't care, but the program coordinator/assistant may bug you to make sure you get thing set up quickly and pass next time.