Some door post moldings are prone to going "wild" on you if you touch them with the wrong procedure and/or product, I have recently found..
Awhile back I had fits with an '07 BMW X5 with black Shadowline Trim.
One side, after I just washed it, went ugly and would absolutely not respond to anything I did until I got this one product called "TM 1000", that is made to "restore, clean, beautify, protect exterior bumpers, painted trim, etc..."
It made me think that perhaps some trim pieces are possibly not made in the same country they used to be made in, and unfortunately, they respond differently than OEM pieces.
The reason I found out about this product was because I had a suspicion that this side of the vehicle had been painted, and when I called the Body Shop that would have been the most logical one for BMW paintwork,, they immediately knew the vehicle, knew they had replaced that trim and said to use this product on it, and it worked, thank goodness..
You should first try to get the Dealer to replace, not fix, replace it, and hopefully it will be ok and stay that way.
If it's painted plastic, you really have to be extra careful on it if you are trying to buff it, so that you dont run through the paint..
I know others have said that you can just buff the heck out of it with Meguiars 105,etc., but I am especially leery of doing something like that on a Client's vehicle, unless I am ready to pony up the $$$ to replace the trim piece if it fails to respond or gets uglier..
Good luck; thank goodness your vehicle is new and is probably still under warranty.
Im going to stop by that BodyShop soon as I have time and talk to them about this issue with Shadowline trim, since I detail alot of Bimmers and have never seen this issue until that X5.
Dan F