Plus there appears to be a plating bubble to the left of the mintmark. This is caused by zinc corrosion under the copper layer... possibly the plating was split or microscopically holed, allowing air/moisture to get in.
As already mentioned it is Die Deterioration. This forms on the fields. (which moves back for me a new start of the cent single squeeze dies to 1987) It will appear on the right side of the date, left side on LIBERTY and on the tops of the motto. (depending on how the die wears)
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