That small portion you're talking about is the Beastmen Leader you're keeping alive..... That bar goes to full when you kill him.
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I have never seen it go full when we have killed the final beastman because the fight automatically ends and the cutscene begins, which gets rid of all the hamlet related huds. Even when you kill all the beastmen before the next wave comes there is still that small amount of red left.
I've seen people talking about some mobs coming with different weapon types than the rest of the wave but I haven't had a chance to look into it. Might have something to do with keeping them alive.