The bug
Upon trying to grow a 2x2 tree (spruce, dark oak, or jungle), sometimes nothing will happen. They won't grow naturally or with bonemeal. It is not exactly known when exactly this happens and when it does not happen.
If there are no blocks next to the saplings and you try to grow a 2x2 tree underground, the saplings will disappear and the tree will grow on the surface instead.
This issue is not related to the heightmap corruptions from 20w17a, as it is reproducible in a brand new world.