Cannot make Polyline dynamically attach and move between two point entities

CesiumJS
2

Polyline positions can be defined using an array of references, and this loads and renders with no issues.

See this sandcastle code that renders as expected:

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

This also works with a time-varying position, and the line updates as expected:

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