The terrain gets cut at Y=256.
Features like ice spikes or naturally generated structures work fine. 

This is not a feature request as it works fine with negative coordinates.

To reproduce:

1. Download the datapack
2. Load a new MC world with it
3. Find a biome called Extreme Mountains Tier 3
4. Find any Extreme Mountains biome with Tier 4 or higher
5. → It is completely same

I discovered this bug while I found The Tier 4 being exactly the same as tier 3.
It's because the overworld noise setting preset allows terrain only up to 256. If you modify the noise settings, it will work fine, so this is probably intended.