Dear John,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
As per the description shared, I understand your concern and to my knowledge, the error message User not in the directory generally occurs when the external users with whom you have shared are not added as a guest users in your organization.
In this situation, I would like to request you to connect with the IT admin of your organization and ask them to check whether the external users having the problem are shown as guest users in your organization at the Azure Active Directory:
Steps for the client IT admin to check whether the your account is added as a guest:
Access https://portal.azure.com with IT admin account details> Search for the Users and click on it> Search for your user email address and see whether you are available
If no, please ask your organization IT admin to invite the external user email address as a Guest users to your organization and ask your external users to accept the invitation to see whether they are able to access the shared content.
If the external users are already part of your organization as a Guest users but still having the same issue, I am sorry to convey that the problem should be further investigated by the related development team, where they can collect the HAR logs while reproducing the problem and perform further investigation.
Note: I have checked the service health dashboard at the Microsoft end, where there is no related incident found.
So, I request you to connect with your IT admin of your organization (How do I find my Microsoft 365 admin? - Microsoft Support) and raise a new service request from the Microsoft 365 admin center so that the related development team will connect with you and investigate the issue further.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: We apologize for redirecting you to related development support team because the moderators in this community are limited with the resource for further investigation and to get a quick resolution to your problem, we requested it.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia