You'll have to ensure that you've selected the modified invoice. Let me guide you how:
From the Gear icon, select Customform styles.
Locate the invoice.
On the ACTION drop-down, select Mark default.
Once done, you'll have to reopen the invoice and update its template.
If the template is correct, you can run some steps to isolate what causes the issue.
To start with, try opening QuickBooks on a private window. This is the safest place to verify any system-related error. You can use these keyboard keys to open one:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Microsoft Edge: Ctrl + Shift + P
Safari: Command + Shift + N
If it works, you can go back to a regular browser and clear its cache. A piled cache can also be the reason for some unexpected concerns on the browser.
You also have the option to switch to a different supported browser like Google Chrome, Mozilla Firefox, Safari, or Microsoft Edge.