We have only tested this with zombies, however, whenever they fall from lethal height on an extended piston they take no fall damage. They are falling on the arm of the piston, not the body. I cannot time a screenshot of this very well unfortunately.
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It sounds like you're using a trident killer. Could it be that the mobs are getting hit by the trident just before they land? In that case it would be the damage immunity mechanic and not the extended piston that is cancelling fall damage.