You are unable to fly forward with elytra and slow falling when flying into a block then turning straight around. The only way to reset this is to look down and then straight and only then would you continue to fly forward. While you are in the state of constantly falling down, if you observe yourself in F5 the player model would start to flicker too.

Steps to reproduce:
1. Drink a slow falling potion
2. Activate Elytra
3. Fly against a wall
4. Turn straight around to where there is space to fly. You would not be moving forward.
5. Observe yourself in third person. The player’s model would start flickering.
6. Look down and then look back up again. You would start flying like normal.