Some biomes are just flat, or they are straight up broken, using the mod versions, and NOT the datapack versions. Happens on my modpack, and just with the 2 mods, and mod required for it.
-MC Version is 1.20.1, newest versions of Terralith and Tectonic, mod versions for both, Singleplayer, Forge, 1.20.1, happens on both with just Terralith and Tectonic, and in my modpack, (latest version, and a few others)
my modpack: https://www.curseforge.com/minecraft/modpacks/canpacificcraft