External Sharepoint Invitee need help recovering Microsoft account

Hello,

A year ago, a user in my organization shared a file on the corporate Sharepoint tenancy with an external recipient using the external user's work email address. The external recipient does not have any personal nor work Azure, Entra, M365/O365 accounts.

When the original, first invite was sent, the external guest registered with their work email address and was able to access the file. Recently, this external user has lost access for an unknown reason. The external user recognized this by going through a Microsoft login loop, where they are receiving an message that their account is not recognized.

Internally, we tried to resolve this by removing the external user and then re-inviting the external user. But a strange symptom is that each time we enter the full email address, it seems the system recognizes the users and adds only the username (first part of the external email address). So when the external user receives a new invite, it only recognizes the username only and no longer recognizes the full email address. Though the username is recognized, Sharepoint is no longer able to use the previously password and the account is locked. Unfortunately, the external user is not able to recover the account for external access to our Sharepoint, as they are not able to provide enough quantity of information to Microsoft; Microsoft denies account recovery due to lack of information quantity.

My question is, how does the external user make a request to Microsoft that the previous account used to access our tenant Sharepoint get deleted? Recovery is not possible at this time. Since this external user received a Sharepoint link, this user is not a user on our tenancy, so how would I be able to access his Microsoft Account (using his email address as the login credential)? When we re-share with this user, why on our Sharepoint tenancy it does not use the full email address and only remembers the username?

This may be confusing, but I have tried it clearly as possible. Any help from anyone who has come across this similar situation, would be greatly appreciated.

Thank you!