Is it a mod compatibility issue?

No

If it's a mod compatibility issue, please provide the link(s) to the incompatible mod(s)

No response

Minecraft version

1.20.1

Describe the issue

The collision box for determining whether the player should be crawling, sneaking, or standing is 0.1 blocks smaller in all directions than the player's hitbox.

So, if the player is crawling and goes under a sideways chain elevated 1 block from the ground, they start sneaking and clip into the chain. The player also starts standing from inside a crawlspace if they're on the outermost 0.1 blocks, clipping into the ceiling.

Fixed in the next version.