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 [Mod] Marcono1234 in this comment.