Wall of Flesh

How does Wall of Flesh's eyes lifting and descending mechanics work. I know that if there are some blocks beneath or above them, then eyes lifts, or descends. However in some cases it works very weirdly. And can you just try to explain me these algorithms?
 
If I remember correctly the eyes will try to stay above anything that's at least two blocks thick, it just usually happens to be Ash Blocks
 
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