User selected a different invoice template using the Preview [Choose Template] option in the Invoices screen. However, Core does not remember this choice and reverts to the previous template next time.
This is the expected behavior in Core. If you change the invoice template in the Invoices screen, it only applies for that one instance or session. If you want Core to remember the template for the project, you need to check the Automatically assign this template to corresponding project option in the Choose Template dialog. You can also set up the relevant template from the Global Settings or Projects screen. Check Core Help for details.