From the documentation of Scene.sampleHeight:
This function only samples height from globe tiles and 3D Tiles that are rendered in the current view.
Does Scene.sampleHeightMostDetailed also work the same way?
It should work even if a tileset isn’t loaded. This example was created to demonstrate clamping to a 3D Tileset even when it’s not all in view:
https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Sample%20Height%20from%203D%20Tiles.html