Polygon vs rectangle curvature

I'm new to Cesium, but I have the following question:

I draw a rectangle and a polygon using exactly same coordinates, but the result seems different, specifically latitude lines curved differently - the rectangle looks parallel to gridlines when using GridImageryProvider while polygon is not.
Longitude line are aligned.
I actually need polygons (with edges not necessary north-south) and need the way to curve the edges correctly. How can I do it?

The code I'm testing with follows

Thank you


Rectangles are drawn to follow rhumb lines so they have constant bearing. The rectangle follows the latitude and longitude lines of the Earth. The lines that connect polygon coordinates are drawn using geodesics, or ‘great arcs’. It’s the shortest distance between the two points along the ellipsoid surface. We plan to add rhumb line support to other geometry types, and have an issue written up here: https://github.com/AnalyticalGraphicsInc/cesium/issues/4000