The Warden has a 'warden_bioluminescent_layer' texture to make certain parts of it glow in the dark. The bottom face of the Warden's right arm is not filled in, whereas the left arm is.
This can be noticed in-game by comparing the bottom faces of both of the Warden's arms.
How to Reproduce
1. Make a layer of barrier blocks.
2. Spawn a Warden above it.
3. Go below the Warden
One side of the Warden's arm is not "glowing".
Both sides of the Warden's arm should "glow".