The input ArcGIS.Core.Geometry.SpatialReference must have an ArcGIS.Core.Geometry.SpatialReference.IsProjected value of true otherwise it will be ignored. Assuming IsProjected is true, the spatial reference is assigned to the underlying projected coordinate system for the CIMMeasuredGrid. The CIMMeasuredGrid uses an ArcGIS.Core.Internal.CIM.ProjectedCoordinateSystem which cannot be assigned in the public api directly.