when blocking with a shield in the latest snapshot, it slightly moves your other arm for some reason.
Regular arm position without blocking
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Position of the arm when blocking. In 1.16 and versions prior, the arm stays in the same position as the one when the player is not blocking.
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I can confirm that this is an issue. This is basically the same behavior as described in MC-225931 but occurs when blocking with a shield instead of eating. Also affects 1.17.1.