Freeze on common mobs - potential bug?

I'm playing a monk (Invoker) specialised in cold damage. Many of my hits freeze mobs but.. I've noticed that sometimes right after a mob receives the status it becomes invulnerable to my attacks. I can keep hitting him but the only thing that changes is the freeze duration bar below his HP bar. Have you ever experienced anything like that? Is this a bug or some kind of mechanic I am not aware of? (like +1000% armor when frozen)