Check the TradersVariables.txt
> <ZombieCleanupTimer> 30.0
// Time in Seconds Interval of Zombie Cleanup in Safezones (smaller Values can cause bad Server-Performance).

He doesnt use the zombie base class in the deletion of zombies. So if you added new zombies that arent vanilla its likely they wont be deleted.

Now I dont know for certain if this works with modded zombies but the fastest workaround which actually safes some resources would be editing the zombiespawns with the economy editor in Dayz tools. Otherwise its not an issue with this mod but rather the config of the other mod adding these infected has to be edited.