The bug

It appears that natural llama spit cannot be detected using commands. This might be related to the decreased life time of the spit and the loss of its Owner tag (?).

How to reproduce

1. Set up a repeating command block with the command

   /execute as @e[type=llama_spit] run say Found!
   

2. Verify that it works

   /summon llama_spit
   

3. Spawn a llama and make it spit, e.g. by spawning wolves or by angering it
   → The llama spit is not detected