Example "Airfield" on Chernarus:

• download / verify vanilla Dayz data
• set additional spawn points for showcase:

<zone name="InfectedArmy" smin="0" smax="0" dmin="1" dmax="1" x="4775" z="9684.6" r="50"/>
<zone name="InfectedArmy" smin="0" smax="0" dmin="1" dmax="1" x="4764.14" z="9701.6" r="50"/>
<zone name="InfectedArmy" smin="0" smax="0" dmin="1" dmax="1" x="4788.14" z="9664.6" r="50"/>

• in Events.xml, set for "InfectedArmy" event

saferadius to 25 (to ensure, they are spawning - can be any value)
distanceradius to 0 (can be any value)
cleanupradius to 100 (can be any value as well)

• Start Game
• Go to roof from building at: 4662.54 / 9587.02. Look at the Airfield.

Expected Behavior:

• Zombies should spawn. (if you turn around, and look at the barracks, the other InfectedArmy Zombies are spawning and roaming

Actual Behavior:

• Zombies are not spawning on the Airfield. Unless you get too close to the actual zone(s). (or approach from another side than the barracks.
• If Zombie has spawned and you go back towards 4662.54 / 9587.02, Zombies start to despawn shortly.

Note:
Other InfectedArmy Zombies are still present on this area.

replace the additional spawns on the airfield in "zombie_territories.xml" from "InfectedArmy" to any other type, for example "InfectedFirefighter"

• Zombie starts spawning. The Issue is not present

adding Army Types to "InfectedFirefighter" Event, for example: "ZmbM_SoldierNormal":

<child lootmax="5" lootmin="0" max="0" min="50" type="ZmbM_FirefighterNormal"/>
<child lootmax="5" lootmin="0" max="0" min="50" type="ZmbM_SoldierNormal"/>

• Zombie starts spawning. The Issue is not present

For some reason, this does not work the other way around.
Adding Firefighter Types to "Infectedarmy" Event, for example "ZmbM_FirefighterNormal", does not work

• Zombie do not spawn. The Issue is present