As the title says. Once you get high enough, you just can't launch yourself forwards using fireworks. It seems to be happening right at the build limit. I tested it with just Elytra Aeronautics by itself (works as expected) and without this mod, just Sodium alone (works like vanilla) but as soon as you put the two together, there suddenly appears a limit as to how high you can be to boost yourself with a firework. if it wasn't already obvious, this breaks the mod in a major way seeing as you can no longer reach the heights required to receive the speed boost.

I figured I'd include a video to show the issue (Note that I have Mod Menu installed in the recording to show that just the 2 mods are enabled, but when I was testing earlier I did not have mod menu installed):

The red ring is the build limit. As you can see, the player still attempts to launch the fireworks, it is just delayed until the player falls back below that threshold.