I want to guess that this is intentional or something, as it says that the avian lost their wings a long time ago. But I would think if this was a actual trait then it would have been listed on the traits tab for avian. But it says nothing about Avians not able to use elytras to fly. I even tested this same thing out in vanilla, got an elytra, and drank a potion of slow falling and it worked as planned there. Maybe this is a bug?

I can't reproduce this. Maybe this is an incompatibility with some other mod you have installed?