Only the Main Line of the transaction is displayed on the custom sublist view.
An Item Fulfillment on a saved search shows an item as this is present on the main line. Invoices and Sales Orders do not have the item name listed under the main line.
You can create a saved search and show the * on the saved search. This is the main line.
This is currently tagged as an Enhancement: 165261 -Search > Saved Searches > New > Transaction > "Available as Sublist View" = T. Please allow users to see transaction line items on custom sublist views if Mainline = F.