The real problem is that this PCB has no etchback, positive or negative (PCBs are defective). As far as resin recession, yes the processing probably caused the separation, but once again, the PCB has no etchback.
I would send that cross-section image to the PCB supplier and asked them if this is typical. I suspect that they will realize it is a trap and either offer a crazy answer or admit that their is no etchback and offer to re-run/replace the PCBs.