2010-D Jefferson Nickel - Very Strange Mushy Reverse

Alright, I've had this one in my "" pile for a couple weeks. I've never seen anything like it. The obverse of the coin is completely normal, typical circulated 2010-D nickel. But the reverse looks really mushy, like the die was out of focus or something. It's got the normal amount of gloss and luster that I'd associate with a 2010-D nickel, but it's uniformly mushy.



The following pictures are not out of focus!









My first thought was Struck Through Grease, but the effect is completely uniform, which I've never seen. It appears to be fully struck - all the features are present and more-or-less full depth, just mushy. You can also see depressed "shadows" of some devices like the lettering. Both the high and lowest relief details are affected the same way.

Another thought was that it was struck through a thin late-stage die cap (the reverse die is the hammer now IIRC), but would it produce features this uniform? Maybe struck through some thin, flat object, like a sheet of plastic?

It doesn't look like PMD because there's no discoloration or big scratches or any other signs of abuse. I've also never seen a coin struck from a over-polished or late-state die look like this.

What's going on with this nickel?