Zombie mode boss health bug

Recently whenever I get together a decent team for zombie mode, and we take down the boss's or mini-boss's health too fast, it stays alive for about a minute or so without dying, and leaves it on what looks like 1hp. Is this intentional to stop people 1-hitting, or is it a bug? Is there any way the timer could be reduced if it's intentional, so the legitimate players aren't effected?