When gliding with the Elytra, the rotation of the rendered player head does not match with the position of the cursor. (Direction that player is looking, essentially)
This is problematic for two reasons really. Firstly it looks very strange if looking straight up or straight down while gliding.
And secondly (also arguably more importantly) this completely ruins any use the Elytra has in map-making with block models. The block models in the head slot of the player will over-rotate instead of looking where the player is actually facing. Example: https://i.gyazo.com/7404fe66590678393da985c54cbfc0b0.mp4
I propose that the player head should only rotate 180 degrees as that is the maximum pitch you can change with your cursor. It would also be beneficial for the player head to rotate with the body too.
[EDIT] Here's a video I made which better illustrates why this is such an issue: https://www.youtube.com/watch?v=ZcklKh9hZuI
I hope you can fix this.
Possibly has the same root cause as MC-90598.