This is considered a bug because the intent was to mimic a Console Edition feature but it was implemented incorrectly.

A special feature for Creative Mode allows players to gain height while holding the jump button. This is to make the elytra much more fun to use as you almost never need to land and can fly at ludicrous speeds forever. The feature was added to Bedrock during the 1.2 betas to match the Console Edition. However, one part of it was done incorrectly that almost entirely ruins its purpose. I'll try to explain the bug the best I can.

On Console Edition, when you hold the jump button while flying, your current vertical momentum is stored but not applied. Instead, a constant vertical gain equal to the speed of flying up in fly mode is added. No matter what way the player faces, the vertical gain will be the same. The vertical momentum changes will be stored but not applied, e.g. facing straight down while holding jump will still move the player upward but gain downward momentum. The momentum isn't applied until the player releases the jump button.

Horizontal momentum, however, is still calculated and applied normally. This means you can be looking down-forward to go very fast in a certain direction while not losing height. It adds a great feeling of speed to elytra flight.

The way it's handled on Bedrock Edition instead applies a constant upward momentum to the player. Instead of allowing the player the fly forward and go up, they will stop mid-air and float up slowly. When facing down, it can take a very long time for the momentum to actually change your direction. This takes away almost all the fun from elytra height gain with jump in Creative.

TL;DR: On Console Edition, the elytra height gain in Creative works a lot better than Bedrock, which is trying to emulate it.