When I shift right-click a Strider with fungus and then unshift and walk any amount of distance, I can shift and tp back to where the Strider was at. I was able to tp back to it 1000 blocks away. My assumption is that the game thinks the player is mounting the strider, but it knows that shifting cancels it, but still sort-of mounts the Strider, so when I shift it unmounts the Strider and when you unmount (Me just guessing) teleports the player at the Strider.