When you crouch in creative mode to move downwards when you lift up on the crouch key the player does a double bounce effect that is very noticeable and annoying. Here is a gif of it happening. https://gyazo.com/3079067111567a58a3cd5edd0849f64d
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This can be easily fixed by just making the player not being able to sneak if currently flying.

Another solution is to have a slightly different animation for crouching while flying, one that doesn't have you sink into the ground and doesn't change the eye height as much.
edit: added a few words.

The player sneak while flying is super useful for testing stuff. I'm sure there is a better way to fix this than removing features.
With MC-147290 being fixed in 19w14b, the fix for this issue has been reverted.

I'm still noticing this issue in pre-release 3. It's a bit less frequent, but still noticeable.
I'm not able to confirm that, seems to be fixed for me. When you're landing, you start sneaking again which might be perceivable as a "bounce", but that is most probably intended.
That's always been like that, you're on the ground and releasing the shift key causes you to un-crouch