It is not clear that the enableCollision option for Cesium3DTileset affects the behavior of HeightReference.CLAMP_TO_GROUND. Neither mentions the relationship in the documentaiton

enableCollision -If true, allows collisions for camera collisions or picking. While this is true the camera will be prevented from going in or below the tileset surface if ScreenSpaceCameraController#enableCollisionDetection is true. This can have performance implecations if the tileset contains tile with a larger number of vertices.

HeightReference.CLAMP_TO_GROUND - The position is clamped to the terrain and 3D Tiles.
 
I was only aware of this once it was pointed out by a team mate

This also came up on the forum - https://community.cesium.com/t/clamptoground-for-google-photorealistic-3d-tiles-broken/39720