Comment by smeyer

I changed my last name when I got married, and at the time I had a work email address that looked like oldlastname@company.com on the company's Google Workspace. Given that oldlastname was no longer my name, I changed my email address to newlastnmae@company.com.

This worked fine in Google services and some of the many work applications using Google for auth, but some of them were using the email address as a global identifier in a way that broke down when I changed my name. The services that migrated successfully were using a more stable identifier that persisted despite the address change.