If you attempt to block an arrow with a shield after it is shot but before it hits you, it will ignore the shield, and damage you. No durability damage is done to the shield. This may be due to the game checking for the shield when the arrow is fired, not when it hits. The video clip will be using optifabric (my hardware's not fantastic, and OBS doesn't help.) but this occurs in vanilla regardless.
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That is because of the warm-up delay of 5 game ticks (0.25 seconds). If you right click, you will not immediately block, this warm up delay was removed in the combat test snapshot.
Duplicate of MC-100949
My bad, I searched to specifically.