Like the title indicates, the Enchanted Forest Biom from "Oh the Bioms you will go" (BYG) generates in Atum2 on top of Sand Dunes Biom.

Map View:

I use the Latest Forge in MC 1.16.5
Atum 2
BYG
Terraforged
All on their latest version.

Could you try installing Blame? It´s a mod that checks if mods are not registering their ConfiguredFeatures. Atum is doing this correctly, as of version 2.2.4

Download here: https://www.curseforge.com/minecraft/mc-mods/blame

Should be fixed by bb61b74

I had a similar issue with Better End Forge, where the Dust Wasteland biome replaced Dried River biomes. but it has been closed because there wasn't much that could be done about it, though it might be different this time