When the world is converted from 1.12.2 to 1.13-pre10, the items in some double chests are transposed between the top half and bottom half of the double chest.

It depends on the direction the chest is facing. It only affects double chests whose locks are facing East or North. Chests facing South or West are not affected.

To reproduce:

Start a superflat world in 1.12.2.
Place down four double chests, one facing in each direction.
Place a few items in each chest. A line of items arranged diagonally works well.
Save the world.
Load the world in 1.13-pre10.
Open the double chests. The items in two of the four chests (the ones that face East or North) will be transposed.