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I have opened several chests, some I had personally placed and filled, one was a naturally spawned treasure chest. When these personally placed and filled chests were opened things inside suddenly disappeared and the treasure chest that had not been opened or discovered yet was completely empty.

These chests also often appear invisible, showing only their outline when hovered over.
Closing the world and reopening it does not return the items to the chests.

This happened in a newly created survival single-player world, the only experimental features I had were the minecart changes.