This had been a problem with my 2019 M3 LR for months. It happens because the driver's butt hits the B-pillar interior trim when getting in, particularly likely because of the "easy entry" position of the seat. Tesla should have used more of the plastic panel retainer clips for that trim, given it's vulnerability.
Over the summer it came loose again (photo 1) and fist pounding it didn't work to make it go back. I finally got around to looking in there and found two reasons why. The first is that there is a plastic spike just below one of the upper retainer clips, possibly intended as a guide spike, that had drooped down in the summer heat (photo 2) and wouldn't go back in its guide hole, preventing the nearby clip from reaching its metal hole in the pillar. I could have just cut the guide spike off, but didn't want to modify the panel from stock in case I needed to take it to the service center, so I straightened it out.
The second problem is that another one of the retainer clips had fallen out and was found on the floor inside the B-pillar trim space. The molded plastic U-shaped socket for this clip has its opening facing downward (photo 3), which allowed the clip to fall out. This IS a design defect, albeit a minor one. It's likely that this clip falling out resulted in less overall holding power of the remaining clips on the trim, making it more likely the trim would come loose again. I used silicone sealant around the base of the clip when I put it back to keep it in position.
If others have this problem, look for the missing clip inside the trim space on the floor. Of course, keeping your butt from hitting that trim will help prevent recurrence also.
