The Last used timestamp is only set on subsequent authentications after the user has been registered. Once the user logs out and logs back in (triggering a new authentication flow), the MFA method's Last used field will populate correctly with a timestamp.

Important clarification:
Even though Last used shows null after registration, the user is still required to complete the MFA challenge during the registration process. The null value does not indicate that MFA was bypassed — it simply means the "Last used" tracking starts from the second authentication onward, not the registration itself.

Regarding registration abuse:
If you're experiencing self-service registration abuse, the null Last used field is not the cause. Consider these mitigation strategies:

1. Identity Pre-Verification (available since v1.62.0): Use the Identity Pre-Verification Using Email Verification Form feature to require email verification before completing registration.

2. Third-party identity verification: Implement a self-service registration lambda that calls an external identity verification service via REST API before allowing user creation. Fideo is a documented example, but any identity verification system with a REST API can be integrated.

3. Rate limiting and CAPTCHA: Consider additional anti-abuse measures at the application level to prevent automated registration spam.