Local time of a radioactive isotope is set to 0 at the end of the track with G4OpticalPhysics registered. For more details, please see the User Forum post (https://geant4-forum.web.cern.ch/t/local-time-of-a-radioactive-isotope-is-set-to-0-at-the-end-of-the-track-with-g4opticalphysics-registered/11891)
Thanks for reporting this. I'm able to reproduce it and am working on a fix.