Nice of you to join this thread, @FMW.

I also appreciate you for taking the time to perform some troubleshooting steps to isolate this. Allow me to impart some insights about subtotal items.

The option to calculate the subtotal item's quantity is only available in the Enterprise version. You'll want to upgrade your QuickBooks Desktop (QBDT) to have this feature, or you can look for a third-party app that has this setup.

You can simply go to the App Center in your QBDT or visit our Apps for QuickBooks Desktop page for reference. 

I've also added this link that you can read for guidance. This contains details on how you can personalize your invoice and make it unique: Use and customize form templates.

I'll be around if you need more help managing your items and invoices. Just leave a comment in this thread and I'll get back to you as soon as possible. Take care!