Create Subscription /users/{id}/events failed with ExtensionError in some users

Question

I'm trying creating a subscription for /users/{id}/events, but failed with ExtensionError in some users.

Here is a detail about what I did and what occurred.

What I did

1. Create a user which authorized with Calendars.Read permission.
2. Check validation endpoint is implemented correctly by Postman.
   
3. Request to create a subscription.

   const response = await axios.post<any, any>(  
         'https://graph.microsoft.com/v1.0/subscriptions',  
         {  
           changeType: 'created,updated',  
           notificationUrl: `http://nitte-functions.jp.ngrok.io/push_calendar_channel_notification_ms`,  
           resource: `/users/{my_id}/events`,  
           expirationDateTime: '2022-07-18T00:00:00.000Z' // set with in 3 days  
         },  
         headers: {  
           Authorization: `Bearer xxxxx`  
         }  
       )  
   

4. It returns this error in some users, but succeeded in other users. 'ExtensionError'
   'Operation: Create; Exception: [Status Code: ServiceUnavailable; Reason: Target resource '00037ffe-3e39-7873-0000-000000000000' hosted on database '8bf41b6b-0cba-4703-9cf6-0f90da806005' is currently on backend 'Unknown']'

How can I fix this?