Just got back yesterday, not sure if its a bug or not but Forest of dead trees 4 and CM2 seem to have very low zombies spawn.
I believe it is due to the new source having a different spawnrate than the old source (old source used custom spawnrates, new source uses the original gms spawns)
It's likely we will keep it this way for the foreseeable future. What we may think about changing though is hp/exp ratios for certain training locations to make them more viable.