The bug

Llama Spit entities are always destroyed when coming into contact with anything other than air.

How to reproduce

(Make sure player is in survival, and difficulty is set to easy, or higher)

1. Load the following structure into your world:
   llama_spit_test.nbt
2. Stand on the red block:
   
3. Run the following command:

   /damage @e[type=minecraft:llama,distance=0..10,limit=1] 0.5 minecraft:player_attack by @p
   

Observed results

The llama spit particle will collide with the tall grass, and be destroyed. Because of this, the player will not be damaged.

Expected results

The llama spit would go through the tall grass, and damagethe player.

Notes

While the current behavior does technically make logistical sense-- that being, llama spit in real life would colide with anything-- it does have an effect on gameplay. It is much harder for llamas to be able to spit on the player if the projectile will almost always hit something.