In the Oracle most of the Orcs vanished during combat.
My whole party was stealthed right by the entrance, attacked from range to start the battle, and maintained stealth during the whole first round. Laid down a spike growth on the bridge then backed up out of arrow range so the orcs would have to jump across the gap and come down the bridge. The cheiftan made the jump and one other orc I killed before he had a chance but all the others just disappeared. I thought maybe since they couldn't see me they wandered off out of the battle. But once the battle (now with just the cheiftan) ended, they were nowhere to be found. Not sure if it's because I went out of their range, because of the AoE effect, or some combination of the two.