When looking from top/down you can see a wrinkle of the membrane, when looking down/top it looks like a metal plate. I think it is a bellow which can expand/inflate with low pressure and shrink/deflate with high pressure.
It is used for depth sensing but can also be used as an altimeter. So it would make sense.
I can confirm its a membrane/bellow and the small wrinkle/fold in the membrane is within spec and all inspected Ultra’s on display at an Apple Store in Amsterdam had it.
Interesting I definitely do not have that on mine. My hole just goes back until there is a metal of some kind across the entire back. The black around the inside of the hole is just shadows ... Otherwise smooth and flat in the back there