That's definitely an oops by FileMaker when they released the template!
Here's how to fix it.
Select Purchase Order from the drop down to navigate to this layout without using a script.
Select View as List.
Now return to detail view and click the button again.
Thanks you so much for your suggestion! I gave that a go and for a minute it seemed as though it might work but unfortunately it hasn't solved the problem. Do you know another way I might be able to get around this issue?
It worked for me. Don't see why it didn't work for you. You can also update the script that you click to bring up this layout to specify View as List.
Or you can go to that layout, enter layout mode, select Layout Setup... and use the Views tab to restrict the view to only List for this layout.