Using the 1.16.1-1.3.6 version of the mod, with forge 32.0.108.

While sprinting, activating an Elytra with any speed boost enchantment will let you fly for as long as the elytra has durability with a fixed velocity (faster with each speed boost level). You can gain altitude at will with no penalty, akin to creative flying but with more speed. Letting go of the W key fixes this issue (not sprinting anymore fixes the fixed velocity as it normally is) This issue doesn't happen if you activate the elytra from walking speed.

demonstration : https://youtu.be/JrcS3YTu3E4
-same issue happens with only the elytra on, forgot to take my armor off for the recording