0.21.0 has major issues, you should switch to this version or if you're on 1.16 remove WAC!
Unfortunately my workaround for 0.21.0 didn't work as well as I had hoped at all. It worked well on my local test server with no lag and latency, but not in a real environment.
In 1.16, the onGround flag of the Spigot API is broken (someone else opened an issue for it here
https://hub.spigotmc.org/jira/browse/SPIGOT-5810). I tried to work around this by manually parsing the movement packets, but as soon as a little bit of lag comes into play this completely breaks down. To fix this, I tried stuff like storing the packets in a queue and reading them when receiving the events, which *might* be able to mitigate lag but comes with a hundred other problems.
So long story short, I'm reverting to 0.20.4 (I gave it a new version number to prevent confusion), which removes 1.16 support again, and until this issue is fixed there will probably not be 1.16 compatibility for WAC. I'm really sorry about this, but there's not much I can do.