Slime blocks pushed up by a piston sometimes don't make the player jump up, instead he remains standing on them as if they where a normal block. This happens at random but only if the player is moving or sprinting while on the slime block itself.

The issue is very unpredictable and I thus can't be replicated 100% of the times; the best way I found to test it was to build some simple jump pad desings and see if they work properly. Keep in mind that you have to sprint while you are on them. In attachment are 2 of the design that I tested this with.

As far I could see this bug has been present since the snapshot 18w44a and is still present as of 1.14 pre 2. I don't know that the cause of this bug is but I suspect this has to do with MC-138105, a bug regarding pistons that was fixed in 18w44a.