azure - Why app-only tokens doesn't work for beta API in Outlook Mail REST API? -

i'm trying use beta version of ms outlook mail rest app-only tokens requests doesn't work expected.

for example, folder hierarchy synchronization behaves v2.0 , doesn't work app-only tokens.

the app-only token works request. what's error get?

to call request app-only token, need specific user instead of me in document request below:

get https://outlook.office365.com/api/v2.0/users/userprincipalname/mailfolders 

and app-token, use code below:

   public static async task<string> gettokenbycert(string clientid, string tenant, string certthumbprint,string resource)     {         string authority = $"https://login.windows.net/{tenant}";          x509certificate2 cert = certhelper.findcert(certthumbprint);         var certcred = new clientassertioncertificate(clientid, cert);         var authcontext = new microsoft.identitymodel.clients.activedirectory.authenticationcontext(authority);         authenticationresult result = null;         try         {             result = await authcontext.acquiretokenasync(resource, certcred);         }         catch (exception ex)         {         }          return result.accesstoken;     }