Error getting thumbnails with app token, works after calling once with user token

[Rens Braspenning]

I'm trying to get thumbnails for new documents in SP online using an app token. This always fails at first. But when calling it once with a user token, it works with the app token. It seems like the graph api only generates the thumbnail when a request is received with a user token.

Steps to reproduce:

1. Generate app token from https://login.microsoftonline.com/{tenantid}/oauth2/token
2. GET: https://graph.microsoft.com/v1.0/drives/{driveid}/items/{itemid}/thumbnails/. This gives the following response: 
   {
   "error": {
   "code": "UnknownError",
   "message": "",
   "innerError": {
   "request-id": "640819f7-8a64-4e58-bed1-44aeef214859",
   "date": "2018-01-30T10:27:58"
   }
   }
   }
3. Generate user token
4. GET: https://graph.microsoft.com/v1.0/drives/{driveid}/items/{itemid}/thumbnails/ using the user token. This gives a valid response.
5. GET: https://graph.microsoft.com/v1.0/drives/{driveid}/items/{itemid}/thumbnails/ using the app token from step 1. Now the response is also valid.

I hope this is not by design? :)