In physics_lists/builders/src/G4EmExtraPhysics.cc the contents of the functions BuildSynch() and BuildMuonNuclear() are swapped. Therefore if one uses the command "/physics_engine/tailor/MuonNuclear on" synchrotron radiation is activated, while "/physics_engine/tailor/SyncRadiation on" activates muon-nuclear interaction. This bug is newly introduced in Geant4 9.1 and obviously very easy to fix.
Thanks for the report. The bug is fixed.