See the screenshot below. Even if it works as intended, it looks so ugly. I'm using 32x32 pack, but that doesn't change the UV and the model of the elytra.
Linked issues
is duplicated by
relates to
Attachments
Comments

Attached screenshot with default resource pack. Seems like it is the same stretched. Looking carefully it seems like it is exactly 2x thicker that it should be.

Is this also an issue in 1.11?

Oh, I forgot to reply, sorry. Yes, this happens in 1.11 too.

Confirmed in 1.16.1 and 20w30a.
Can confirm in 20w49a.
Can confirm in 20w51a.

Affects 21w03a
Example for resource pack:
[media]On the side were stretched because its done.
It can be seen, the pixels are 4x4, instead of 2x2.
For default resource packs it seems a bit different pixels.
(NOTE)
Red x-crossed = Not affected as bug ❌
Red border mark = Affected as the bug ✔
Can confirm in 21w05b.
Can confirm in 21w06a.

In 21w08a

Relates to MC-212747

In 21w11a
Can confirm in 21w17a.
Can confirm in 1.17.
Can confirm in 1.17.1.
Can confirm in 1.18.
Can confirm in 1.18.1.
Can confirm in 1.18.2.
Does this occur without the resource pack?