When model (assembly or part) contains relation
mass=mp_mass(")
and when config.pro option mass_property_calculate is set to automatic, mass properties of sub-assembly and parts will get calculated and assigned each time the assembly gets regenerated.
In WF3 that actually ment that you need to regenerate the assembly at least twice and save it for the mass properties to get populated among the subassemblies and parts.
Hope this helps.
Regards,
Jure