I was able to replicate this. Seems to be an issue with your dedicated server spawn point being set inside the mall. If you move it outside the mall the zombies no longer despawn inside...
Perhaps it is coded so that when players log into buildings that have spawn points in them (or in this case an servertest.ini spawnpoint) they are cleared for spawn protection. So in your case the whole mall gets cleared anytime new players come, or old players who logged out inside the mall log back in or someone dies.