This has just happened on my Office 365 account with the new sign in experience - I can no longer change verification option and enter a code rather than approve or deny based on notification.
This was enforced on Windows Phone/Mobile it appears as notifications no longer work - however users using the code over notification on iOS or Android will most likely be getting the same lockout as they cannot provide 2factor anymore to successfully login.
What I have had to fall back to:
It appears the new sign in experience multi factor request needs to add the verification code to the new experience.
When I signed up to the new sign in experience it provided the new sign in but reverted to the old screen for 2factor authentication. Now they have updated it so both parts are the 'new sign in' experience it appears they have forgotten about users that log in with a verification code.
So if you're testing the new sign in experience, ensure you aren't using code verification.