This is quite weird. In my multiplayer server (1.17 Vanilla) I had a bug where a tree spawned in without any blocks underneath it. I have attached a short clip showing it. This was not a tree that grew with a sapling, this spawned naturally. What's odd is when I went to a single player and re-entered the same seed and went to that location, the tree was not there.

Seed: [3285541509627547661]

Coords: [-15 71 457]