I think your coin may be this type of Struck Through error coin. https://www.error-ref.com/struck-through-smooth-viscous-material-grease-oil/
Sure is odd, no matter how it happened. If it was PMD, which was my first thought, I'd think the rim would show signs of damage.
I acquired it slabbed. I look out for these type of unique NGC attributed mint errors on ebay. I just don't remember how mich I paid for it.