Mobs that shoot/throw projectiles will sometimes shoot their projectiles whithout being physically there. i cant hit them but if i place a block where i think they are, the block disappears after a split second and returns to my inventory. if i close the world and rejoin again they will still be there, but visible and interactable this time.

i've had this happen with pillagers, skeletons, and trident drowned, so im geussing it affects all projectile shooting mobs