shield glitches with certain mobs (more on description)

When I'm blocking with a shield, for certain attacks like from a vindicator, after he hits my shield my shield stops working for a cooldown. Is that intended? But the big thing is if it's intended, then why when players, skeletons' arrows, or zombies/other monsters hit you, it still works, there is still NO cooldown? This is really confusing if none of this is a glitch then can someone explain to me how the shield cooldown works and with what mobs?