Hello Matthew Dugal,
This is expected behavior — you cannot re-register MFA for your own account from the admin side. To do this for an account, you would need to sign in with another Global Administrator account.
Seeing "Default sign-in method (Preview) – No default" is also expected.
If you want to register a new MFA method for your own account, you can do so directly from: https://mysignins.microsoft.com/security-info
From there, you can add a new MFA method as needed.