The sharp corner of line disappear when clampToGround is true

the sharp corner of line disappear when clampToGround is true

below linking is the codes shared by cesium sandcastle

cesium sandcastle example

when clampToGround is false
image
when clampToGround is true
image

Question1: is this a bug?
Question2: if i want no sharp corner when clampToGroud is false
what shoud i do?