If view KML files with a terrainProvider then geometries appear to be shifting as you zoom in/out and pan around.
If no terrainProvider is enabled in Cesium (tested with v1.18) then all examples are displayed same as they appear in Google Earth.
The problem is clearly seen with these examples http://kml-samples.googlecode.com/svn/trunk/morekml/Lines_and_Paths/
In particular these files illustrate the issue:
Enable terrainProvider and drag/drop then drag/drop these kml files onto the map :