[2019] Assassin: is this a feature or a bug?

The clones are invincible, but they disappear after being hit. You can tell if a clone hits you, with seeing bugs on your screen!

His week spot can be found in the middle of his chest (where the fleshy part is showing), hitting that stuns him and prevents him from cloning as well.

You did get 2 hitmarkers at one point before he cloned and attacked, he probably had a tiny bit of HP left.