While I am not sure what led this to happen, I have found that leaving and rejoining the world does not seem to do anything. Furthermore, it seems that crouching while the shield is not in your off-hand plays no animation when I believe it should.
Update: It seems that restarting the game does not fix this issue.
Update Two: The bugged animation does not always play, although it does the majority of the time.
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We're tracking this issue in MCPE-116703 and MCPE-116736, so this ticket is being resolved and linked as a duplicate.
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