The bug

If you look at a parrot, ghast or the ender dragon through a spyglass, the respective advancements are not granted if the eyes of the mob are blocked.

Steps to reproduce

1. For better consistency, summon any of these mobs with NoAI. For example, I will use a Ghast:

/summon ghast ~ ~ ~ {NoAI:true,Silent:true}

2. Place blocks in front of the Ghast's eyes (if this is difficult, switch to spectator and look for the blue line that you can see inside the Ghast with F3+B)
3. Look at the mob with a spyglass
4. The "Is It a Balloon?" advancement will not be granted even when you are looking at the parts of the Ghast that are unobstructed

What is expected instead

On step 4, the "Is It a Balloon?" advancement should be granted when the player is looking at the Ghast through a spyglass.

Related issues

When the player is looking at the Ghast's hitbox through a solid block while the Ghast's eyes are unobstructed, the advancement is granted even though it should not be.

Causes

These problems are caused by the advancements using the looking_at predicate, of which the intended behavior isn't to test if the player is looking at a mob, but instead, its current intended behavior is to test if the mob can see the player while the player is looking in the mob's direction.

Getting the advancements to behave correctly would therefore need them to get special behavior that is different from the looking_at predicate, or to change the behavior of the looking_at predicate from the previously intended behavior.