My 8" bed is unpainted except for about 3/4" around the edges, and insulated with 1/4" cork tile, with the heater board on the original bolts and springs ( one nylon spacer at the home corner). I have noticed a very similar problem recently at the front edges of the bed, with ABS/110deg bed setting. I haven't done any precise measurement although I could. My main indication is that an abs print of the same stl and otherwise same slice besides the different material profile, shows the issue and pla does not.

Looks to be aluminum bed frame time. Anybody selling a good one?