I doubt it, seeing as all the cavity consists of tapered shapes. A mushroom would leave behind some evidence of being sealed inside, and the mushroom cap structure grows above ground, while spuds grow buried underneath soil. It logistically doesn't quite check out IMO.

What I suspect happens in these kinds of shapes- and I see them fairly often -is that the potato simply expands as it grows, and it just pulls apart at some point of stress, kinda like a warped piece of wood.

Except its an oblate spheroid, so it "cracks" in the center.

This could be some kind of encapsulated material, I don't know the details of how potatoes protect themselves from foreign bodies.

But, in theory, there could be some kinda dead fungus in there, slain by a potato-based void...?