Choosing the appropriate template when generating an invoice ensures that the footer you set up is displayed correctly. I'll guide you through the steps so it appears correctly on your end, Tsd.
Once you've configure the footer under the Custom Form style page, make sure to select the Customize option when creating an invoice. Then select the specific template you've created or modified. In my case, I've edited a template and saved it as "Standard", which is why I've selected this option. Please refer to the attached screenshot for visual guide.
After previewing the invoice, you'll be able to see the footer you've customized. Please refer to the example below:
For your references, you can take a look at the following write-up to guide you with utilizing your invoice template in QuickBooks Online as well as on what other information you can add: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Drop me a comment by hitting the Reply button if you have any other concerns with the missing footer of your invoices. The Community and I will be happy to help you out.