I'm not so sure that this is a bug, per se, as a difference in expectations. By one way of thought, seeing someone to be knocked completely unconscious, it's not unreasonable to expect that they'd miss the whole party.
It seems that what you're referring to is something more akin to being stunned. With the advent of less than lethal combat, there are some hints that the stunning effect would make an appearance finally, possibly in this release, but I can't find a quote to confirm. I say that since there has been mention of saps and blackjacks. Knocking someone unconscious, binding them with ropes and carting them away as part of the combat AI has specifically been mentioned as something that is tabled until further development of the thief role by WoT. That means ON PURPOSE. The underlying mechanics have not been specifically stated to be non-present, so may be an emergent behavior soon.
Now, syndromes have an interesting way of getting directly into the data structure of bodies and/or souls. That's how transformations work, for instance. I'm not aware of any entity flag that isn't transparent to syndromes. If not in vanilla reactions, there should, soon we hope, be a way to make a modded syndrome, i.e. and adventure mode reaction, that sets the stun flag/timer. That would probably be closer to what you're looking for.