Can anyone else comment with first-hand knowledge on this? I have not seen it myself, but I have not had bridge battles under the same circumstances evidently, or maybe I just missed it, and assumed my units did the killing with out knowing the 'drop dead' was occurring. My intent in 90% of my bridge battles is to destroy all the enemy forces, and it is usually a challenge to cut off all the escapees... so my attention is often not in the bridge area in the waning parts of such a battle where my infantry/archers are still comfortably banging heads sometimes.
I'm still not clear on something: When the units drop dead, does the battle end at that point? Or does the battle continue, but there is a steaming new pile of corpses where 2 seconds ago there were fighting enemy soldiers? Have you seen it happen to archers? Do you recall just what kinds of infantry specifically died (e.g., warband? phalanx?).
I know that I have routed and chased enemy Phalanx from the bridge without them droping dead with Patch 1.2, totally stock (no mods at all). But I don't recall specifics on others.