Slime Block is unstable in SMP, player glitches inside it when it's pushed up.

When I push a slime block up with a sticky piston while standing on top, most of the time the slime block gets pushed through me and I get pushed to the side.

It can work rarely and pushing the block while doing little bounces on it does help. It seems like it's because of lag but I have quite a decent connection to the server.