The projectile of skulkers are unable to pass through scaffolding blocks. Projectiles of skulkers was able to pass through scaffolding in 1.20.4 and back to at least 1.17, possibly even earlier.

Steps to Reproduce:

1. Spawn an shulker
2. Have the shulker fire a projectile at a scaffold

Here is a possible contraption to reproduce the issue: 

The snow golem aggravate the skulker, leading it to shoot an projectile toward the top scaffold.

Observed Behavior:

The shulker projectile is blocked and explodes upon touching the top scaffolding.

Expected Result:

The shulker projectile should be able to pass the scaffolding.