If the attackers didn't manage to take a single defensive line in a wave, the game ends right there and then. Everyone gets their XP and they can move on to a more balanced round.
The behemoth is good for games where the attackers just needs a little extra push (f.e. only taking 2 points / wave), but not taking a single line is just pointless and annoying for everyone. No one likes playing the same point for three waves in a row.
This shouldn't take too much development work and could probably be implemented at least as a quick-fix in the very near future.