if you fall on the Slime Block from any distance while holding the jump key (default: Space), the slime block will not launch you in the air & will jump like normal (or higher if you have Jump Boost).
I do not know if this is intentional.

I do not know a lot about Java, but I think a fix for this would be kinda complicated, as the game would have to ignore the jump key being held only if the player has fallen more than their jump height can take them (so they can jump without interruption if they jump from one block bellow the Slime block). the game would also have to take into account the level of jump boost the player has or any extra jump height added by items in mods (since the Modding API is something the developers are working towards)