I'm very confused as to how and why I lost a

[Taqwus]

Design decision, not bug.

Attacker automatically routs after a certain point; a while later if the battle hasn't already ended, all remaining attackers (berserkers, mindless and other unbreakables; the unconscious; those bogged down and blocked by a horde of defenders; those simply not fast enough e.g. due to injury, interfering spells and location) -die- regardless of stats.

Blocking a gate is a reasonable tactic for the defender, but not an unbeatable one in that an attacker can bring ranged firepower (magic, missiles) and fliers in addition to trying to carve one's way through. Pan CW might have a hard time massing archers (not too many Sag Carcasses get generated) and battle mages (the emphasis, IIRC, is on Nature, which is wonderful for support but less so for direct damage attacks) , but you can get other mage branches (Lamia Queens, Spectres come to mind) and you do get some heavy inf (minotaur carcasses, and the Ettin Mandragora can be very brutal given decent gear).

The auto-killing of berserkers is something to watch out for. In an SP game, I recently lost a nicely-equipped Tartarian Cyclops who was using the Unquenched Sword; the sword berserked him and immobilized the defenders through fatigue, but there were enough unconscious defenders at the end that he couldn't kill them all before auto-kill kicked in.