When an arrow is fired before the player blocks with a shield, the player will receive the damage carried by the arrow even if they block with the shield before the arrow hits.
This has been tested with skeletons, Pillagers, and dispensers, but not with other players.
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duplicates
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I'm quite certain in saying that this is a duplicate ofย MC-100949. ๐

Thank you for your report!
We're tracking this issue in MC-100949, so this ticket is being resolved and linked as a duplicate.
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Does MC-100949 describe your issue?