Since the area is not discolored, and since any chemical erosion is likely to penetrate the thin copper plating, I'm inclined to classify this as a struck-through error. It's either a grease strike or a closely-related error, an impression of a floating encrustation.
Grease would prevent the shine from happening on that area. There was grease in the deeper recess of the die and a bot on the field that prevented the shine in that area. (there may have been some debris in the grease on the field, leaving it rough)
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