It happened to both zombies and skeletons, so it could be an issue with all hostile mobs who can hold weapons. The bug only happens when they are moving, the weapon returns to the place it belongs as soon as the mob stops chasing the player.
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Duplicate of MC-300136.
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Most likely caused by a mob wielding an enchanted weapon, as both zombie and the skeleton in the screenshot had enchanted weapons, while a skeleton that had a regular bow did not have the same issue