Closed Naga portal - endless stream of Nagas continue

[Ranto]

I'm in one of the orc camps, entered the Naga portal and destroyed the cleared the level, and got given an achievement for stopping the Naga invasion.

On returning to the Orc camp, and making my way to where the old portal was, I find there's two Naga. I think nothing of it and proceed to dispatch them with due prejudice.

However, every time I kill one a new one spawns so that there's always at least two. Is this a bug, or have I neglected to do something? (Naga portal is no longer there as fas as I can see)

[snoop]

I've had the same thing happen with a Fearscape invasion portal too.