What happens:

When your scale is set to a low value (e.g. 0.064), you cannot move on certain blocks while crouching. These blocks include slabs when no collision is directly below them, or the thin parts of stairs. The smaller the block's collision, the higher your scale value can be for this to happen. You can observe this bug with a scale of 0.25 on trapdoors for example.

How to reproduce:

Set your scale attribute to a very low number (e.g. 0.064) and stand on a block with a small vertical collision box that isn't directly connected to another collision box, like an upper slab. Now try crouch walking.