My first thought was a pitted die which is usually more localized than die wear, but since that wasn't an option, and I don't know if this same effect is seen on other areas, I picked "die wear,"
Let's not start that again with Insider! Well, what looks like the 'cratered' areas seem to be raised, and my first thought was corrosion on the dies, but not an option. Next thought was etching (acid) on the dies, but again, the options don't specify which surface the acid etching would be on, so to play it safe, I'll go with 'Worn Die'. But if the Acid Etched option could apply to acid on the die, I'll go with that.
A worn die. Erosion due to extended use or possibly that of an improperly hardened die. My avatar is an example - VAM-1C Edit to add - The "buffed reverses" subset of VAMs is one I actively collect.
I voted worn die due to the location. The movement of metal to fill the incuse area of the die is more pronounced than the larger open areas.
Given the incuse nature of the anomaly I am going with die rust (which definitely isn't too unusual to find on Seated coinage).
Good comments. I can only add the elementary observation that if this damage is on the die, it is on the flat surface only, and the incuse lettering is unaffected. I can imagine the flat surface of the die (which strikes the fields) could suffer damage, corrosion, or rust that would not affect the lettering or devices. It could have been set on a shelf and rusted, or dropped onto polishing residue, and so on. Several of the previous posts seem very plausible. I eagerly await the reveal.
I'm going with acid etched surface. Notice that the anomaly is in and around the letters only (IOW the protected areas of the coin) and NOT in the field below the letters. I think that would rule out a worn die, which would likely exhibit the problem over a wider area. Plus, inside the letters is where the acid used might pool, for one thing, and not rinse off completely, for another. Another giveaway as to pooling of the acid is that the letters seem unaffected. That would also likely rule out a die issue.