All of the immediate areas around the spot of concern seem damage free, it would seem odd that this coin being in nice condition (AU) was subject to
PMD in the one spot, be it corrosion or tampering, or (even acid would be more apt to burn more of a circular spot). I'm thinking the planchet had either a bubble of some kind or an impurity that created the void during the pour and sheeting of the raw stock. IMO