Mob out of bounds, Cenotes map

Cenotes tileset. I found a Bogfelled Slave spawned down below in the water, outside walkable space. The mob was moving back and forth in the water against the cliff wall, unable to path to walkable space.
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As I was writing this and unpaused the game to confirm once more the mob name, he suddenly in clunky movements popped up onto the platform (entered walkable space)