With the new UI changes, the outline of a multi-slot item (example: 3x1) in inventory do not cover the container slot overlay. Additionally, not all item images scale to cover all of the container slots in-use by the item. The result is container shows slots in-use but it isn't immediately clear which items are using those slots.

For an experienced player you can work around this but for a new player, it is not intuitive.

The preferred solution is to have a border around the item in container and you can still see the number of slots it takes up underneath. Something like this (just showing the multi-slot items):