Line 135 of G4Nucleus.cc is
theTarget.SetMass(targetMass*G4Neutron::Neutron()->GetPDGMass());
The mass of the target is in amu (I believe), so it should be multiplied by amu_c2 (931.494028 MeV) and not neutron_mass_c2 (939.56536 MeV) which is returned by G4Neutron::Neutron()->GetPDGMass(). The difference is small but still…
Thanks for checking. Best regards,
The TMX coder