The bug

Fireworks are a lot more finnicky to launch yourself to fly with the elytra than in previous snapshots and versions. I am not sure when this issue came about, but I know it is since 21w05b, since in that snapshot it was fine. I didn't play much since then until the latest snapshot, so I didn't notice the problem until now.
Previously, if you take off by jumping and launching yourself, you would be able to fly, no questions asked. But now it is much harder to take off. Sometimes, you'll fly like 5 blocks in the air and then you'll stop flying for no reason and just fall. Sometimes, you'll use a firework, which implies that you are flying, but you won't fly at all. It is almost impossible to get out of the water with the elytra now. And sometimes you just totally glitch out while doing it, which I can't really explain in words, but when it happens you'll know what I'm talking about.

Steps to reproduce

1. Create a world in 21w05b or 1.16.5 and try to launch yourself with the elytra without falling any blocks, just from taking off from the ground. Note the relative ease of doing so.
2. Create a world in 21w15a and try to launch yourself. Notice that it is much more difficult and often leads to glitchy behavior.

Additional Information

It seems that in 1.16.5 it was very easy, in 21w05b it was slightly more finnicky, and in 21w15a it is the worst.