Why it is possible to see sales order with zero in order quantity in plan inputs if these are closed

Summary:

In plan inputs when you see the sales order type, you can see lines that have zero in the order quantity, but this lines are closed in the order, so why this lines are shown in plan inputs?

Content (required):

This lines should not be shown in the plan inputs, because these orders and lines has been closed.

Version (include the version you are using, if applicable):

21D