My potato had a mushroom shaped internal cavity

This is referred to as hollow heart.

It usually indicates as shortage of available calcium in the plant as the tuber grows. Basically the same thing as blossom end rot in tomatoes, peppers etc.

Some varieties are more prone to developing this than others.

The Internal cavities are usually a result of poor growing conditions.

It's just two internal holes that happen to look like a mushroom together. I've had potatoes with one of those holes before.

I believe they are caused by lack of water or just happen sometime.

Here's an example that doesn't look like a mushroom, and had been cooked: https://www.eatortoss.com/whats-up-with-this-freaky-hole-inside-my-baked-potato/

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...?

It happens sometimes with large potatoes.

Hide-a-shroom.