I don't know what else to say. It's hard to explain.
EDIT: He's not going through the blocks, it just looks like he is.

How to reproduce:
1. Go to an ocean monument in Creative.
2. Get the Elder Guardian on the top level of the monument to look at you.
3. Go outside, using Spectator.
4. He's looking at you, and he appears to be sticking his "head?" outside the monument.

May take a few tries.
This most likely is caused my the elder guardian hitbox being slightly smaller than the Elder Guardian itself, and also, because the hitbox doesn't rotate along the Y-axis.
I can understand if you don't fix it.