When player's generic.step_height is greater than or equal to ((Collidable block's hitbox total height under the player)+1.5) blocks, they can't move while sneaking.

Steps to reproduce:

1.Create a normal superflat world (4 blocks thick) and run the command /attribute @s minecraft:generic.step_height base set 5.5

(5.5 can be replaced by a larger number)

2.Press shift and W(Sneak button and move forward button).

Observed results:

You can't move forward when you are sneaking.

Expected results:

You can move forward when you are sneaking.

Code Analysis is in the comments.