Due to a critical issue with passive mobs not spawning, a hotfix has been released to remedy this.
This change was done due to how Minecraft's passive mob generation works, as it will always check the topmost biome of the world to determine which passive mobs spawn when a chunk is first generated. Since The Frontier does not allow mob spawning, this caused passive mobs to be absent upon world generation, and they will only spawn slowly over time with a very limited mob cap.
The removal of The Frontier is unfortunately necessary for now, but I will look into other ways of solving this issue in the future in a way that allows me to keep the desired effect.
Report CliffTree issues here

A vanilla-styled worldgen overhaul, with new biomes and touches to existing ones. Short for "Cliff under a Tree"