The bug

When pushing a slime block into a player who is flying, they will stop flying and fall; this does not happen with other blocks such as stone or honey. After looking at update history for slime blocks there does not appear to be a reason for this change.

To reproduce

1. Place a sticky piston facing upwards
2. Place a slime block on the top face of the sticky piston
3. Then 3 blocks up (or two air spaces) place a solid block
4. Stand on the slime and fly upwards until you hit the block
5. Place a pressure plate below you and observe as you fall

Observed behavior

After the slime is pushed up, you fall down (your flying is cancelled).

Expected behavior

The player would remain flying after the slime is pushed into them, consistent with all other blocks.