When using a datapack to create a custom assortment of random configured features in a custom biome, the file will not be accepted by minecraft and will simply generate nothing.

To Recreate

1. Use the datapack I have attached. It includes a custom biome which is a copy-paste of SlicedLime's code for giant_tree_taiga except it calls custom:testing_trees instead of minecraft:trees_giant_spruce. custom:testing_trees is an exact copy of minecraft:trees_giant_spruce.

2. Generate a world using the "single biome" setting and chose "custom:test_taiga" as your biome.

3. Create world and witness not a thickly treed Mega Taiga but instead a rocky hilled plain.