Hello @Andrew Hudson,
Thank you for connecting offline over team's call.
As we observed during our call, you were unable to register your guest account with the Microsoft Authenticator app in the tenant.
Upon troubleshooting, we found a workaround to register an MFA method for your account. By signing in with another global admin in the tenant, we added an SMS authentication method to your affected account via the Azure Portal. After adding the method, you were able to log in successfully without any issues in both tenants.
As discussed on the call, I will check internally regarding the Microsoft Authenticator registration failure for your account and will share any updates with you via email.
I hope this information is helpful. Please feel free to reach out if you have any further questions.