The bug

Players cannot sneak exactly 0.0~1.0 block outside world border, whether to start or continue sneaking.

Doesn't affect inside or more than 1.0 blocks outside world border.

This happens whenever the coordinate of worlderborder is integers.

To reproduce

/worldborder center 0.0 0.0
/worldborder set 16
/fill -16 100 0 15 100 0 bedrock replace
/tp -9.1 100 0.0

• Try pressing Shift and go near the border.

/worldborder center 0.1 0.0 

• Try pressing Shift and go near the border.

Expected result

The player would crouch.

Observed result

The player does not crouch.