It's impossible to block if you are aiming at a mob with less than 2 blocks from you.

To reproduce:

1 Equip a shield

2 Get close to a mob (less than 2 blocks)

3 Aim at the mob and press the block button

While creating this issue also found that if you have a shield in your left hand and no item in your right hand and try to block while aiming at a Fence you also won't block.