Lamination peel on that area. Nice example of a woody that shows that the color is not on the surface, but all the way through the planchet. Unlike the 70's-80's copper examples with the rolling lines, that wear off of the design, called roller lines. What is the weight? If looks like the planchet was reduced in thickness a bit?
The weakness on the reverse is not the affected area that could be affected as a pre-lamination. So it peeled off post strike.