I was playing on a private server with some friends and while I was exploring the world, two things were happening. First, in all the Exterme Hills and Extreme Hills+ biomes I passed through, llamas refused to spawn, until I go below Y=30. Then, below that Y marker, llamas started spawning in the caves. Other mobs like chickens, cows, sheep, and pigs also spawned in the caves but they still spawned on the surface. On the server, it's completely vanilla, no plugins or mods.