In an email from Apple this week:
In an effort to keep your account more secure, two-factor authentication will be required to sign in to your Apple Developer account and Certificates, Identifiers & Profiles starting February 27, 2019. This extra layer of security for your Apple ID helps ensure that you’re the only person who can access your account. If you haven’t already enabled two-factor authentication for your Apple ID, please learn more and update your security settings.
I've separated my personal and developer accounts so wasn't too sure what to do. Always a good thing when others experience the issue too:
I ended up going with temporary macOS user which I eventually deleted.