Hello caosu,
Thank you for posting your query on Microsoft Q&A.
The username and password are verified against the identity provider (IDP) to which you are sending the authentication request. If the IDP doesn't contain the username & password for the specified user account, the authentication fails.
For the Guest/External users, the credentials are stored in their home tenant and not in the guest tenant. Guest users have an attribute named altSecID which contains the information about the home tenant of the user to facilitate the authentication
Please confirm if you have added your Microsoft account to the tenant as guest user and verified