Ever since the crouching animation was added this bug has existed: https://bugs.mojang.com/browse/MC-159163
It's because the server sends back the visual starting and stopping crouching animations when the player crouches, but information takes time to travel and it is trivial to crouch and uncrouch before the server sends those packets back. This causes a visual desync on the client end.
Don't believe me? (Speedbridgers know this all too well)
Load up a world or join a server after version 1.14 and tap your crouch key very quickly. (as in press crouch and quickly uncrouch, not spam it). You will notice and extra crouch animation much of the time.
BTW
This mod borrows code from sneak tweaking here:
https://www.curseforge.com/minecraft/mc-mods/sneak-tweak
so if you want more functionality to tweak how crouching works, then go there. I just wanted to fix a bug not a bigger mod with dependencies.
