Yesterday I visited github.com, entered username and password, and 2FA code, but kept seeing "Two-factor authentication failed".
Needing access (and getting nowhere with 2FA), I used an account recovery code. These are supposed to grant access in the event a 2FA device is lost.
But the recovery codes did not work (!!).
A Stack Overflow post shows others had a similar experience:
https://stackoverflow.com/questions/55507076/why-does-github-2fa-recovery-codes-fail
(13 upvotes and 9k views at the time of posting).
If you want check whether your recovery codes work, try logging out of github.com and when logging back in instead of entering a 2FA code, select 'Use a recovery code'. Curious to know if they are broken for anybody else.