I consider the auto loss to be a bug, rather than something that should be a suggestion. If you're properly knocked out (e.g. through a bit of "reinforcement" when out, I don't think there's any need for a rope.
It can also be noted that the auto loss description isn't correct: I had a character with a low pain threshold who was knocked out by a lucky arrow in a fight, woke up, went to pick up spare gear, returned to the tile where the fight took place, zoomed in, and the fight resumed. It seemed everyone (henchmen, dog, and robbers) were frozen in place during my character's travels (not very far, as the homestead was just a few kilometers away), The resumed fight was won, by the way...
Thus, your robber most probably didn't kill everyone, just took advantage of a momentary lapse of observation to perform a few impossible feats of stealth.
At first I assumed it was a bug as well, but now it's been like this in the game for so long that I imagine it has to be working as intended. You're right, the fight may have been frozen in time, as I believe I simply quit out of the game after that rather than try and track the tile back down, haha.
The reason I suggest rope/cord to bind an unconscious victim is to make it an in game mechanic. Basically, if I don't have the option of instantly stripping people down and teleporting them away whenever they get knocked out, NPCs shouldn't either. It's also more realistic, as being knocked out doesn't work the same in real life as it does in movies, where people stay conveniently unconscious for however long the plot demands it. If you're ever out for more than a few breaths you've likely suffered permanent brain damage. It also offers a convenient moment to trigger the robber resolution script (stripping valuables, moving PC to X nearby location), by sparking it as soon as the binding is finished. However, like I said, I'd be satisfied as well just changing it so that it doesn't trigger until hostilities have completely ceased for at least 2-3 ticks and a robber is next to the PC's unconscious body.
Perhaps the problem is with plucky robbers who, for some reason, fight to the death rather than run when they're losing?
I'd be fine with robbers fighting to the death, just not with my character being magically teleported away naked from a fight that's still ongoing.