Let's consider the mass in SolidWorks. If we want a property valued with the mass, this property must be of type "String".
Because this property is a string in SolidWorks, WGM allows to map this property to an attribute only if the attribute is of type String in Windchill, on CAD Document.
On the side WTPart, the mass attribute is of type "Real with units" so it can be used in facets search and some WTPart can have a mass without having an associated CAD Document (therfore an alias attribute cannot be used).
Because there is a type mismatch between the attribute mass on CAD Document (String) and the one on WTPart (Real with Units), the build operation doesn't propagate the value.
My suggestion is an ability to add custom logic for the attribute mapping (during the build) between CAD Document and WTPart (in this case, the custom logic would be a cast from String to Real).