Maybe someone else can figure this out since I can't.

Added a bit of code to the floating block renderer to sync the client and server positions for entities riding on the blocks and it fixed the issue!

so it's not as bad as it was before, but the jitter is still there.
When I moved the code to my GitHub fork it came back. It works every once in a while though so that's a plus!

im tempted to just remove this behavior if it can't be made to work well by release.

Planning to remove in the port to 1.18.

Apparently they can now also get players stuck while floating. Setting noGravity is a bad idea.

Floating blocks seem to need a complete refactor as they seem to be broken.