Some want to show it. Some do not. So you get to control it how you prefer.

There is now a toggle in your Business Center that controls if you want Billing Items to show on Flat Fee Services that have previously been billed but are still being worked. By default these WIP Billings WILL show on the Bill, but specifying "Yes" for "Ignore WIP for Flat Fee Services" will remove those from displaying on a Bill.