The bug
Llamas have an incorrectly sized hitbox until they are reloaded.
It looks like they might have the horse hitbox (not as tall as llamas but wider).
Steps for reproducing (via spawn egg])
Suffocation damage
Spawn in a llama in a 1x1 hole with solid blocks as the walls
Observe that it suffocates, although a llama is 1x1 and should fit in the space
Block collision
Spawn in a llama and try to push it against a wall (nudging/piston/water)
The llama cannot be fully pushed against the wall
This issue relates to MC-107869 / MC-50367.