Deltatoken for Calendar Events requiring elevated sharing permissions?

Question

Hi All,

We are an external party trying to interface with an Azure App created with the scope: Calendar.ReadWrite.Shared

To authenticate, we use delegation using a service account to the users, this access has been granted as Author.

To track changes in a user calendar we perform the call:
GET /users/{id}/calendarView/delta?startDateTime={start_datetime}&endDateTime={end_datetime}

The GET call works and we receive back a skip token.

But when we try use the skip token, or delta token we receive a 404: Not Found.

This behavior was strange, so we asked to change the permissions of the user account to delegate Read and Manage permissions (Full Access) through the o365 portal.

Once this was done, the skip token and delta tokens worked without any issues.

We have also tried giving the Editor scope, with no success. At this point, we are trying to determine how we can delegate calendar specific access, whilst maintaining access to the Events; delta function.