While playing adventure mode, hold both the left click and the right click button.
Then smash instantly smashable blocks such as grass.
Try it on a couple of blocks.
The arm animation will no longer display for instant smashable blocks.
When moving to a non-instant block, the arm will work again, but will return to its bugged state when you move your view to a instant smashable block.
Linked issues
duplicates
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adventure hand
Duplicate of MC-6315 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.