Hi Emilio,
Good catch!
Indeed, I can see that there are a few erroneous branching ratios in the decay products yields database used by FLUKA. The XR and Auger/CE yields are too high with respect to what data suggests and this would cause the surplus of low-energy electrons you are seeing. We have identified all 8 nuclides/isomers to be patched (among which 83m-Kr); they will be corrected for the September 4.3 release.