We are Production orders from Production planning tool. Production Planning tool is creating Production order for Closed Sales order also. As it should not create a Production order for closed Sales order.
I was not able to replicate this scenario in my test account. It is only fetching open sales order, and not the completed ones. Please elaborate your use-case, so that we can support you better.
Please see the below screenshot where SO-00001-1, SO-00012-1 are closed sales order but it showing in Production Planning tool.