This is very interesting. I bought my 2020 X3M certified pre-owned and this exact plastic rivet was missing. On mine, there is obvious signs of where it was pried off and left scratches.
Coincidence?
To rectify it, I bought a general kit off of Amazon. It’s this one:
GOOACC 240PCS Bumper Retainer Clips Car Plastic Rivets Fasteners Push Retainer Kit Most Popular Sizes Auto Push Pin Rivets Set -Door Trim Panel Fender Clips for GM Ford Toyota Honda Chrysler
https://www.amazon.com/dp/B07L6FJGR4...MN7WFFRTQAZ0JZ
I forget the exact river within that kit. I just found the correct diameter, but the length was too long. I just cut the last 1/4” off and got in there nice and snug.