Camera produces unexpected transition at negative heights

Here's a [Sandcastle](https://sandcastle.cesium.com/index.html#c=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) showing this issue around the Dead Sea, where the terrain has negative height. Try clicking "fly to position 2". Notice the camera will fly high up in the air, rotate slightly, then fly back down.

The expect transition here is for the camera to just rotate slightly and not do this extraneous transition.

This only seems to happen when doing camera flights at negative heights.

Start by running the linked Sandcastle example around the Dead Sea and click “fly to position 2” to reproduce the camera transition at negative terrain heights. Trace the camera-flight behavior involved in that example and verify that the flight only rotates slightly, without rising and descending through the air.

Written by the indexing model from the issue text.