I don't like my mod being called "random" but thanks for using my mod anyways.
Well, the reason this bug happens is that the server sends entity pose packet to the client.
When a player sneaks
Client sends entity pose packet(start sneaking)
Server receives entity pose packet
A player stops sneaking, and the client sends entity pose packet(stop sneaking)
Server sends entity pose packet(start sneaking)
Client receives "start sneaking" packet, even though the player has already stopped sneaking
Client stops sneaking 1 tick later
Client receives "stop sneaking" packet
Confirmed for 1.14-prerelease-5
I can see the error using Chrome developer tools.
The error is as follows:
Status Code: 500
I tried to vote from Japan so It might be a timezone issue...? I'm not sure though.