When you are computing the interpolated points using sampleHeightMostDetailed, then they should not be on the ground. There has been an issue at Terrain with multiple layers loading higher LOD tiles inconsistantly · Issue #11312 · CesiumGS/cesium · GitHub where it sometimes returned wrong results, and some forum threads indicating that this issue - even though it was closed - may not be resolved in all cases, but I’m not aware of actual test cases or investigations here.