Multiple ground polygons results in disappearing on camera change

Sandcastle example: https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/index.html#c=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


Browser: all modern browsers

Operating System: Macbook Pro (Retina, 15-inch, Mid 2015)

Problem: Multiple ground polygons with some distance between them results in them occasionally disappearing when the zoom level and/or camera angle change. Doesn't seem to happen unless multiple polygons are added and they are not near each other.