Skulk sensors do not detect projectiles landing, only being thrown. You can test this by throwing a snowball or other projectile at a skulk sensor from far enough away that throwing does not trigger vibrations. When the projectile lands on the skulk sensor, it does not light up or activate.
I am not sure which versions this affects, so I guessed, but I am not in the beta, just using the caves and cliffs toggle in the most recent non beta update.