The bug

The player's hitbox when using elytra is too far back, leading to odd collisions. It's currently too difficult to make hitboxes accurately follow the entity's orientation (MC-91376), but the box could at least be closer to the center of the player's body. The current setup is very problematic when precision flying is needed.

This is also the case with the new 1.13 riptide animation. Observing this misplacement in 3rd person mode is harder to spot, since the animation itself is obscuring the white hitbox. However, when you look at the character in the survival inventory, it is obvious that the model is outside of the black box and obscuring the inbentory slots, just like the elytra animation. 

The new swimming animation on the other hand, has the optimal hitbox location. The hitbox is closer to the center of the player's body, meaning that you can see more of the player's body part when colliding on a wall. The model fits a lot nicer in the black box of the survival inventory, making inventory management a lot easier.