I'm pretty sure that some stone or copper ore did once "sink" through a troll-made hole in a deposit, whereas my character could stand on solid ground at the bottom of this 1-2 meters deep hole. The hint is that I could see the "dropped item" sparkle passing through the ground but autoloot didn't work, like when items have drown in water far below the surface. Sometimes, there were no sparkle at all, as if the item had sunk too far below, or it had despawned.

Could this also apply to gravestones, making them invisible or at least non-interactible?