Since the start of the experimental 1.18 snapshots, some biome variants were removed from generation due to biome placement being now independent of terrain. This includes hills/plateau variants of deserts, forests, badlands, taigas, jungles, etc.
However, savanna plateau biomes can still generate despite being identical to savanna biomes, only exception being llamas not spawning on regular savannas (which they do in bedrock edition). This also could be messing up with biome placement.
This biome was initially removed on the first experimental snapshot, but then was re-added on the second one, even though this was never mentioned in the patch notes.
How to reproduce
Create a world.
/locatebiome savanna_plateau
❌ You will find this biome, not different from regular savannas.
Linked issues
is duplicated by 3
relates to 2
Comments 5

But why would it be the only variant to be able to generate if the other biomes' variants were removed? Because they're also able to generate on hills or plateaus without the need of a variant.

Can confirm.
This might be intended because savanna plateaus spawn different animals than regular savannas.

It's the only reliable place to get llamas within the new world generation now since extreme hills are very rare and small
I'm pretty sure this is intentional, savanna plateaus are similar to normal savannas but in hill version.