Since i haven't seen an explanation, and I'm itching to make my first post, I'll tell you what I found with mine. Mine is a July 2018 Model 3. I had the same issue. the left side of the base was not clipped onto the base correctly, leaving the cover loose. With this cover loose, it allowed the foam to expand a little and every time weight was applied the foam would rub on the edge of a metal frame and would shave a little off of the foam. It shouldn't take service long to fix. I fixed it myself. I had to unbolt the seat though, I could not get it attached with out unbolting. There is a black plastic channel that is sewn to the material that clips to a metal frame. I wish I had snapped a picture, but did not. Get it fixed when you can.
Brian