> it’s in the soft tissue on top of it

Can anyone with a medical background confirm if this is a thing, or folks are just noticing old undulations in their skulls?

You shouldn't need a medical background to know that having something press on a spot for a couple of hours will leave a depression in your skin.

You have probably fallen asleep on something patterned or folded and have it leave an impression on your skin before: This is no different.

Other places it happens: Watches that are slightly too tight or have ridden up an arm. Glasses arms pressing against your temple or behind the ear. Tight socks after a day wearing them.

It's not a medical problem. It's just general physics.