Steps to Reproduce:
Enable the Update 1.21 Experiment;
Enable coordinates;
Grab a wind charge item;
Look down and shoot while paying attention to the height.
Do the same thing, but sneaking.
Observed Results:
You get to the same height regardless of whether you're crouching or not.
Expected Results:
Sneaking allows you to get higher.
Notes:
Should your summary say that they [do] deal extra knockback to crouching players, not that they don’t? That seems to be what your steps and video show.
This issue may be a result of the server-side player position being eye-level, and therefore intended or a “won’t fix.”