for a datapack concept I altered the way acacia trees generate, and it turns out this only affects the naturally generated trees, not the ones from saplings. this applies for both random tick grown saplings as bone mealed saplings.

How to reproduce

1. make a datapack folder
2. change the file <data pack folder>/data/minecraft/worldgen/configured_feature/acacia.json
   change config.trunk_provider.state.Name to diamond_block and save
3. zip this datapack and put it in a new world, make sure it is enabled
4. load a new savanna biome, you should see diamond block trees
5. grab an acacia sapling and place it down
6. grow this sapling either via bone meal or waiting, this doesnt grow diamond trees

NOTE: step 1-3 can be replaced with downloading the provided zip