Terraria – Why do swamp things stand still

I was was heading to the jungle when a Solar Eclipse happen. I hadn't beaten Plantera yet so it occurred naturally. They were killing all my NPCs so I tried to defend against the monsters as fast as possible.

When I died to a Vampire, I noticed a Swamp Thing standing still doing nothing, and then walking off. I thought it was because I died when it was on the screen so spotted one quickly and tried it again, but it kept on moving even after death.

I could not reproduce it within the time I had left of the Solar Eclipse. I went onto a world where Plantera had already been defeated so I could trigger a Solar Eclipse with one of the Solar Tablets the world had.

I turned on my recording software in hopes I could spot a Swamp Thing standing still.

After 10 minutes of nothing, I found it:

Upon further examination, I thought maybe he was standing still because he was against the wall, but there is a clear gap between it and he is able to clear the wall with a single jump when he walks forward.

This only happens with Swamp Things, no other enemy.

Any ideas as to why this is happening?