If you switch the selected item in your hotbar, and the item you were holding was nothing (empty slot) and you switch to nothing (empty slot), the player's hand (first-person) moves down and back up. The hand should move down and up when the selected item changes, not when it doesn't change.
Here is a video showing it (it is on youtube because the file is a large size)
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However, this issue has been closed as a Duplicate of MC-86619.
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