No information on the wiki that there should be a delay for the shield to work before it can block attacks, this leads me to suspect that it may be a bug. However I have heard other people say this was the intended purpose. This delay leaves just enough time for a mob to attack you even if you were blocking as they attack.
If this is it's intended purpose, I understand, but I'd like some information backing this up.
Duplicate of MC-100949.