Guest User Receives 401 "accessDenied" Error When Accessing Shared Files via Microsoft Graph API Despite Valid Authentication #2518

Refernce similar to this in Sharepoint: OneDrive/onedrive-api-docs#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."
}
}