I found a WILD fluctuation in terrain gen in a Cretaceous dimension, with the top of the colossal plateau stopping at a suspicious Y=256. This bug isn't gamebreaking by any means, but it does look strange, especially on sides with sheer cliff face exposed.

There is also snow that follows exact chunk boundaries on the side that slowly ramps up to Y=256, though both sides are in yxian_snowy_forest biome.

The mega plateau has yxian_snowy_forest and yxian_snowy_mountains biomes.

Mod version 2.3.1, compiled from source without edits.