Dear Lex,
thanks a lot for your report and for your analysis. I went back to the history of G4BraggModel.cc and apparently the line that you mention was introduced on 2024-11-08, as a result of a bug fix by @civanch : “fixed problem of 4.12 MeV mu+ range reported in the Forum #12312”, i.e. this post. I just made a quick check and looked for the first occurrence of the line you mentioned, but didn’t verify if this line changed the previous logic. I will give a closer look. If you have the chance to have multiple Geant4 versions installed, it would be great to confirm that this is (or it is not) the source of the change.
Ciao,
Luciano