Guest User Receives 401 "accessDenied" Error When Accessing Shared Files via Microsoft Graph API Despite Valid Authentication

Refernce similar to this in Sharepoint: https://github.com/OneDrive/onedrive-api-docs/issues/1039

Pattern of failure:
Initial API calls fail with 401
After manually visiting the file through SharePoint web UI, API calls work temporarily (1-4 hours)
API calls then fail again after a period of inactivity
Web UI access requires device code authentication flow
What works:
✅ Basic Microsoft Graph calls (/me) succeed
✅ Accessing files through SharePoint web UI works
✅ Token refresh succeeds and produces valid access tokens
What fails:
❌ Microsoft Graph API calls to /shares/{share-id}/driveItem

Issue Type: Microsoft Graph API Authentication
Affected Service: SharePoint Online for Business
User Type: Guest User (External/Cross-Tenant)
Authentication Method: OAuth 2.0 with Refresh Token
API Endpoint: /shares/{share-id}/driveItem

Problem Description:
As a guest user in your tenant, I am unable to programmatically access shared files via the Microsoft Graph API, despite having:

Valid authentication tokens
Proper API permissions granted
Ability to access the same files through the web UI

Detailed Symptoms:

API calls fail with 401 error:

{
"error": {
"code": "accessDenied",
"message": "There has been an error authenticating the request."
}
}

Aucun fichier source ni test n’est indiqué. Commencez par reproduire la requête /shares/{share-id}/driveItem avec le token renouvelé d’un utilisateur invité, puis comparez-la avec /me et avec l’accès depuis l’interface web SharePoint, en utilisant l’issue SharePoint liée comme contexte. La tâche est terminée lorsqu’une cause documentée et une résolution validée de la réponse 401 récurrente sont disponibles.