Can not choose Cesium world terrain as base terrain

Dear Cesium support staff,

Hello! When I upload the raster terrain model (id: 870146) to the Ion, I can only choose the mean sea level as the base terrain, which causes a shift on the z-axis (height) when I align the 3D tile (id:870012) to the terrain.

When I choose the Cesium world terrain as base terrain as shown in the screenshit.png, it went wrong. The merged terrain file is in Lv95 (planimetry) and wgs84 in height.

We have rendered the terrain on the other file with the same process and it works well. (id: 651876) To make sure the error is not caused by our data, we tried to reproduce the 3D tile of 651876 and we can only choose mean sea level as base terrain as well, which also causes a shift on z-axis.

Could you please help me find out what is wrong?

Best regards,