oh, okay, i thought it is a bug, because on build 40 it counts.
If i were a pz developer i would make a full info panel how many zeds killed with which weapon including killed by car and by fire.
There not many milestones in this endless survival, this counter was one of them, you look at it and see that there becoming less and less zombies around (i play with 2x zombies at start, but without spawn), and favorite weapon status does not give any combat bonus, so it is a little bit useless, but that is just my personal opinion.