Thank you for posting this in Microsoft Q&A.
I understand that some of your users are experiencing a sudden and unexpected change in the authentication process for Microsoft 365 (M365), specifically those who typically use a YubiKey FIDO2 key for multi-factor authentication (MFA).
To use a YubiKey FIDO2 key, at least one multi-factor authentication method needs to be registered. Could you please try adding a multi-factor authentication method and see if the FIDO key option appears in the security info page?
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya.
If this answers your query, do click Accept Answer
and Yes
for was this answer helpful. And, if you have any further query do let us know.