Using the Shift+Left Click to quickly send an item to the chest is a quick way to free up your inventory. When riding a donkey or mule with a chest attached and the chest is full, instead of returning the next item back to my inventory, like it would a chest on the ground, the item just disappears and is lost.
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However, this issue is a duplicate of MCPE-23531. Please add your vote to that issue, and comment there if you have additional information to add.
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