This was caught by the mod Blame.

Forge 36.1.4
alexsmobs-1.9.0

[04/12/2021 04:20:22 pm]: ****************** Blame Report 1.9.2 ******************
[04/12/2021 04:20:22 pm]: This ConfiguredFeature was found to be not registered. Look at the JSON info and try to
[04/12/2021 04:20:22 pm]: find which mod it belongs to. Then go tell that mod owner to register their ConfiguredFeature
[04/12/2021 04:20:22 pm]: as otherwise, it will break other mods or datapacks that registered their stuff.
[04/12/2021 04:20:22 pm]: JSON info : {
[04/12/2021 04:20:22 pm]: "config": {},
[04/12/2021 04:20:22 pm]: "type": "alexsmobs:leafcutter_hill"
[04/12/2021 04:20:22 pm]: }
[04/12/2021 04:20:22 pm]: Biome affected : [atmospheric:sparse_rainforest_basin, atmospheric:rainforest_basin, atmospheric:rainforest_mountains, minecraft:jungle, atmospheric:sparse_rainforest_plateau, atmospheric:rainforest_plateau, minecraft:jungle_edge,
[04/12/2021 04:20:22 pm]: minecraft:jungle_hills, minecraft:modified_jungle, minecraft:modified_jungle_edge, atmospheric:rainforest]