When in a board, if you create a folder “A”, and then in there a sub-folder “B”, then add items to “B”. Now rename “A” to “C”. “B” and all of the items within will vanish.
This is a temporary issue, doing a full reload of the board will show the missing folders and items. Also, it appears that you have to create the sub-folder and then rename the parent folder before reloading the full board to trigger the bug. If you create the sub-folder, then reload the board, then rename the parent folder, everything works as expected.