The bug
Most saplings (acacia, both azaleas, birch, spruce, jungle saplings) cannot grow if there is a block diagonally above it. This does not mangrove propagules (sometimes) or any type of mushroom/fungus. In the case of oak saplings, this prevents small oak trees from generating, but not big oak trees.
The growth is also only (generally) prevented by blocks that have a collision box, but it does affect buttons, light blocks, planks, twisting vines and even flowers for some reason. It does not affect the following blocks (list might be incomplete):
Tall grass
Glow lichen
Logs
Vines (sometimes; see MC-193318, affects certain trees)
Wood
Linked issues
is duplicated by
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ampolive
ampolive
The block also does not need to be immediately above the sapling, it could be up to a few (about six) blocks higher than the sapling and still prevent its growth.
ampolive
Attached a test world.
ampolive
I assume this should have the category "World generation" too, like most other tree-generation issues.
It's worth noting that for oak, it does prevent SMALL oak trees from generating, just not big ones. (this is a trick I've been using for a long time when I want big oaks while building).
ampolive
Updated description. Thanks for the info.
Brain81505
Can confirm in 23w03a
Wormbo
Can confirm in 1.21.
Diagonal blocks have been a way to force oak saplings to only grow as "fancy oak", i.e. (usually) large oak trees. I'm reasonably sure this behavior has existed for a long time, and currently a very intentional-looking check in the TreeFeature class. The thing that might not be intentional is that trees that generate with vines or other blocks, that are neither leaves nor logs, don't necessarily ignore those block types.
I thought this was only an issue with azaleas and with non-full blocks, but with more experimentation it turns out all saplings (except oak saplings) do this with any (collidable) block. This might very well be a duplicate, but I didn't find anything using the search feature.
Now I recall that this also happens in older versions. If this is an intended feature, it should be the case for oak and spruce saplings as well.