The bug
Custom trees generate with trimming of leaves at another chunk
And minecraft is spamming in console a message, when it is generating such tree

How to reproduce:

1. Download a zip-file "new_generation.zip".
2. Create your minecraft world. (While creating it press Data Packs and paste in opened folder zip-file new_generation.zip
3. Using /locatebiome find dark_forest biome. Then some trees will be cutted on chunk borders and how I said previously in debug window there will be tons of error messages.

Important thing that this problem appears not only with dark oak trees, it happens with flowers and other trees in this datapack. You can see such errors as with dark_oak trees in debug window.

Example errors:

Detected setBlock in a far chunk [-30, 3], pos: gg{x=-479, y=63, z=59}, status: minecraft:features, currently generating: ResourceKey[minecraft:worldgen/configured_feature / minecraft:dark_forest_vegetation_brown]

Detected setBlock in a far chunk [-131, 379], pos: gg{x=-2089, y=67, z=6064}, status: minecraft:features, currently generating: ResourceKey[minecraft:worldgen/configured_feature / minecraft:patch_grass_savanna]

Detected setBlock in a far chunk [-189, 262], pos: a{x=-3023, y=72, z=4193}, status: minecraft:features, currently generating: ResourceKey[minecraft:worldgen/configured_feature / minecraft:flowers_birch_forest]

There wasn't this bug in 1.16.5 and earlier