when you push end crystals with a piston the crystals go back a half block on the first push. it works how it should on the second push. this bug doesn't happen in 1.9, but when I tested it. it worked in 1.12.2 and up to the latest version (including snapshot)  . I don't know if it works on other updates in between 1.9 and 1.12.2 . to recreate this effect just put down a  pistons (horizontal) . then place a end crystal in front of it. the use a way to activate the redstone (any method works).