The short swords were added when the Blood & Sand DLC came out. I’ve been playing on PVE-C long before that, using all sorts of weapons around my followers, and up until this week I have never caused any damage to them.
It’s not just about the effect being increased, because even if I caused them the smallest possible unit of damage, their health bar would pop up. It never did. Last night it did.
If I had to speculate, I would say that this might be related to the following bug report: