I only found this bug while testing walking and sprinting on top, below, and on the side of one. If you walk or sprint near slime block being pushed in any orientation by a piston, then it will push you up/to the side/down by exactly one block (like any other block being pushed by a piston) instead of giving you increased motion like a slime block usually would.

Also, I tested standing motionless on it/beside it/below it and did receive increased motion.