Error retrieving iCalUId using Microsoft Graph API with delegate access in Outlook (classic)

Question

We are currently developing an add-in for Outlook that needs to support both the classic version on Windows and the web-based version.

To match events, we need to store the corresponding iCalUId's. These are obtained using the Microsoft Graph API with the following steps:

1. Get the item ID using saveAsync (see saveAsync).
2. Convert the item ID to a RestId v2 (see converttorestid).
3. Use our graph API client to get the corresponding iCalUId
   • https://graph.microsoft.com/v1.0/users/{UserId}/events/{RestId}

This process works on Windows as well on the web for creating appointments in calendars owned by the current user. 

But using delegate access the graph api return an 404 error for appointments using Outlook (classic) on Windows, containing the following body:

{
     "error": {
         "code": "ErrorItemNotFound",
         "message": "The specified object was not found in the store."
     }
}

To make it precise, it still works for calendars using delegate access in Outlook on the web.

We appreciate any clarification or assistance provided on this matter.

Best regards,

Lars Schieffer