polylines are curved when create polygon in 2D mode

[TriVectorX]

Intended outcome:
When create polygon in 2D mode, I was expecting straight polylines to be drawn

Actual outcome:
When create polygon in 2D mode, the polylines are curved.

How to reproduce the issue:
Switch to 2D mode by clicking on the globe icon on the upper right corner, then start to draw polygon close to north pole, the polylines are curved.
But when I draw a rectangle in 2D mode close to north pole, a straight polyline rectangle shows on the map

please see the attached screenshots

Version

• angular-cesium@0.66.0