What I expected to happen was...:
Only to tap "Jump" once after being launched (In survival) with a slime block to activate the Elytra

What actually happened was...:
I needed to tap "Jump" twice to be able to activate the Elytra, I think this is because the game looks for a first jump, then a second to activate the Elytra, but being launched by a slime blocks isn't detected as a jump.

Steps to Reproduce:
1. Stand on a slime block above a piston, with an Elytra on.
2. Activate the piston and get shot into the air
3. Either tap "Jump" once to NOT fly, or tap "Jump" twice TO fly