This is a simple bug. When an axe disables your shield, you can still block by holding right-click. It just doesn't show you blocking (To you and to others), but it still blocks.

How to reproduce:
1- Get a player to sprint and hit you with an axe while you are blocking with shield
2- Hold right-click and get the player to hit you
3- You notice the damage was blocked.

Code analysis by marcono1234 in this comment.