There is an error in the calculation of the invariant mass of the two neutrinos in the radiative muon decay (G4MuonRadiativeDecayChannelWithSpin.cc). Instead of using the absolute value of the electron and photon momentum, one should use the corresponding three-vector momenta.

In lines 378ff it reads:
  G4double vmass   = std::sqrt((energy2-
                               (daughtermomentum[0]+daughtermomentum[1]))*
                               (energy2+
                               (daughtermomentum[0]+daughtermomentum[1])));

I would like to recommend to replace this by:
  G4double vmass = energy2*energy2-(daughtermomentum[0]*direction0+daughtermomentum[1]*direction1)*(daughtermomentum[0]*direction0+daughtermomentum[1]*direction1);