Hello Ali,
Thank you for using ArcGIS for AutoCAD! This should be a really simple fix to get those elevations on each contour line within Civil 3D. To display the contour lines within Civil 3D, or most any Autodesk product we need to fill in the Elevation values for each contour. Although this value does not come across when converted from ArcGIS for 2D polylines we should have a column labeled "Contours" or "Elevation" and in this column will be the contours height values for these 2D polylines from ArcGIS.
1) We can simply run an ArcGIS for AutoCAD command, "Esri_ElevateToField", to Programmatically set those elevation values we need in the Autodesk product of choice. Then when you manipulate the drawing to a 3D view the contours will be on their correct elevation.
If by chance after running this command you still have issues please provide screen captures of the Attribute Table from ArcGIS Pro and also ArcGIS for AutoCAD.
This should get you up and running, but if not please let us know.