Open the Lists menu > Customer & Vendor Profiles, the submenu, the Terms List. Edit that Terms to see if it is Define to what it is Named.

Now edit and confirm that Customer has this as their Default.

Edit your invoice Template to confirm no one relabeled the Field you are seeing that seems to be Due date, to confirm they didn't mess up what you think you see with the functional aspect that the program is using.

Now edit their invoice to confirm the Terms is assigned here; that's how the program uses it to set the Due date. Or, you set it manually. Or you select the Terms Manually.

And nothing updates Existing invoices automatically. If you find setup errors or overlooked selections, you also Update the invoices.