Viewer Event Position breaks after enabling, then disabling World Terrain model

It’s possible to get the position of a mouse click on the world via event.position so that an entity can be placed there. For example, here I’m drawing a line from one corner of this octagon to another:

This was taken using the default WGS84 ellipsoid. If I switch to the Cesium World terrain, I can do the same thing again without incident:

But then if I switch back to the WGS84 ellipsoid, the location reported by event.position breaks and it starts reporting incorrect values. Note that in this image I clicked on the same two corners as before:

This does not happen with event.endPosition, but that can’t be used in every circumstance.

Is this an engine bug? Or some implementation error that I can get around?

@dzung should this be reported as an engine bug on github?