Hello, I've just discovereda weird bug were the objects inside chests get it's position changed when optimizing a world from 1.12.2 to 1.13. I can confirm that at least a regular double chest had the items moved because I had on the top right corner four pieces of diamond armor ordered from the helmet to the boots, but after the optimizing the were 2 pieces on the top right corner and other 2 on the bottom right corner, which left me very confused, although I believe no item was missing.

In this other issue there is my world download, so if you converte the 1.12.2 version again you should see the chest over the floating city at around -250, 192, -250 get changed again. It's a regular double chest, separate from the others and near several beacons.

Thanks for your attention and have a nice day.