The separation of the clear from the plastic insert, is most likely the fault of improper prep on the play field before clear is sprayed on. Was the PF cleaned after sanding? Was it Cleared, Inked, and Cleared again? If so did they prep the clear before inking? Did they use plastic prep before clearing over the inserts. Was the humidity up in the paint booth, was the temperature off. The hardness of the plastic, vs. the hardness of the clear, the different expansion coefficients, and the lack of adhesion may have 1 or many contributing factors. Maybe they were rushed, or clears changed, or different inks used, maybe dirty screens, but something in the process has changed. Someone may have signed off on a change control, and we are now seeing the out come of that change.

This is the kind of thing you'd expect from a machine that's got 20 years of use. Not something you've had for a week. Defect in Manufacturing.

Did Stern ever figure out the Tron Dimple issue?

Don't forget some plastic recipes include a mold release agent in them. So Sanding the plastic only causes more release agent to surface. The parts need cleaned, glued in, sanded and the play field cleaned again. I've seen the same thing happen on older play fields that have had the inserts sanded and cleared. You have to properly prep and clean plastics