The bug
Riptide animation goes crazy while sprinting underwater.
How to reproduce
Run:
/give @s minecraft:trident[enchantments={levels:{"minecraft:riptide":3}}]
Sprint underwater and use the trident.
Example
Here is an example in video:
[media]Related issues
is duplicated by
relates to
Attachments
Comments

Confirmed in 1.13.2 and 19w09a
The player seems to spin around on the wrong axis, allowing it to "bellyflop" its way through the ocean.
When I look closely, it is actually a backwards bellyflop since the back is facing forward.

I can confirm this bug in 21w10a, as I was messing around just now, and encountered this bug.

Confirmed in 1.17.1 release