Why would a invoice template change on its own? now the on screen look is different from the printed one?

Hi there, @phill3.

It's possible that the invoice template created was not set as a default. That's why it looks different from the onscreen invoice. Let's go to Custom Form Style and set it as default from there. 

1. In your QuickBooks Online, click the Gear icon and select Custom Form Style.
2. In the Customer form styles list, find the created invoice template.
3. Click the drop-down beside Edit.
4. Select Make default.

Here's an article you can read for more details: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Just in case you want to import custom form styles, feel free to check out this article for more guidance: Import custom form styles for invoices or estimates.

Please know you can continue to reach me here with any additional questions. Thanks for coming to the Community, wishing you continued success.