New UI does not suggest the correct item to be returned. The new UI suggest the item not already invoiced as output or can suggest the item removed from the original items. It happens when order has a change even change value. The "itemsReturnable" is not correct on "api/orders/..invoices/summary".

Create an order with two or more items execute a change removing the itemindex0 and try the return on new UI. The itemindex0 will e a option to be returned.

There is no workaround for now.