Welcome to the Community. It could be something is not working properly with the browser because when you update the previous customer's email, it should apply to new invoices.
Let's check to verify this if it works by accessing your account through a private browser or an incognito window. Then, troubleshoot your main browser once confirmed. Follow these shorcut-keys below to start using it:
Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Log in to your QB Self-Employed account and create an invoice. Update the email to your email address and save it, then try sending it. Once done, create another invoice and check the email. If it's updated successfully, that means you're experiencing a browser issue.
To resolve this, go back to your main browser and follow the steps to clear your cache. This will help eliminate junk files to prevent unusual behavior on the browser. Thus, speeding up your web browsing as well.