From what I can see, tunnels have only an activation threshold on the lower end. A de-activation for bases above the margin is not active it seems. Are you sure there was in the past?
And low level OPs clogging your farming space is nothing i'd call a bug. A nuisance maybe, but if wanna have an easy time farming in low level territory, I don't see why you shouldn't be bothered by dealing with those camps. Or you know, move to more level appropriate areas.