I generated a superflat world  in 1.13.1 with the The Void preset selected (minecraft:air;minecraft:the_void;decoration), and tried to use rocket propelled elytra. The rockets fired while I flew and exploded if crafted with a firework star. However, the elytra did not allow me to gain any velocity or height. I tried this in survival aswell as creative, but to no avail. One thing I did note was that in survival the fireworks crafted with a star did inflict damage to the player as usual. I tried to fly in other worlds, including a custom superflat void with the preset 1;0, and a Tunnellers Dream preset and it worked fine. I logged in to my original The Void preset world several times, yet nothing changed.