The bug
Weird animation of player falling sideways(like dying animation) when trying to go in crawling mode with elytra equipped. After the animation plays out, normal crawling animation starts playing.
How to reproduce
Equip an elytra
Press
F5
to change to third person viewGo near a one block tall space
Double press the jump button to enter fly mode
Linked issues
is duplicated by
Attachments
Comments

The animation that plays, its like player died.

Its in the images named 2,3 and 4
Can confirm that this is an issue in 20w51a. The player tilts to the side in a strange way. I've attached a video demonstrating the problem to clear up any confusion.
Can confirm in 21w03a.
Can confirm in 21w15a.

Duplicate of MC-90143?
Affects 1.21. I would like to request ownership of this report considering the original reporter has not maintained this issue for years now.
What exactly is the bug here?