A post last year was closed citing a solution, however I just checked the most recent version of ROOT (6.24.06) and the same wrong output still occurs.
I believe a fix was suggested by you in the geom/geom/src/TGeoMaterial.cxx but some reason it was not accepted into the main branch of the code?
Can you please look into this. This is a significant error if the decay processes are not properly treating the isomers.
ROOT Version: 6.24.06
Platform: CentOS 7.5