I recently died because of this bug in my Hardcore world and also got in multiple close-to-death situations in a cave when getting shot by skeletons.

The issue occurs when an enemy starts it's attack and you only start blocking while they charge it and not before that. 

For example, when I block and after that, the skeleton charges his bow and shoots, I wont get damaged. However, if I start blocking after the skeleton shot his bow, I will get damage, even though I blocked while the arrow was hitting me.

The same principle applies to the death in my Hardcore world because of a creeper explosion. I only started blocking after the creeper started blinking so I died (even though I blocked during the actual duration).