Creating user messages subscription results in "...backend 'Unknown'" error

Start with the Microsoft Graph subscriptions API documentation linked in the issue and inspect the reported response through the SDK's GraphError.js path. No repository source file or test is identified, and reproduction depends on an unknown subset of users. Done would require establishing whether the SDK exposes or mishandles this service error, with a reproducible case or a clearly documented service-side finding.

Describe the bug

I am trying to create a subscription for one of my customers' users, but it fails with an unexpected error. This seems rare and is isolated to very few users.

Expected behavior

Using the subscriptions API in the SDK results in success.

https://learn.microsoft.com/en-us/graph/api/subscription-post-subscriptions?view=graph-rest-1.0&tabs=http

How to reproduce

Attempt to create a subscription for an impacted user (I do not know what constitutes an impacted user).

SDK Version

^3.0.7

Latest version known to work for scenario above?

No response

Known Workarounds

None

Debug output

Error: Operation: Create; Exception: [Status Code: ServiceUnavailable; Reason: Target resource '<REDACTED_RESOURCE_ID>' hosted on database '<REDACTED_UUID>' is currently on backend 'Unknown']
    at new GraphError (/app/node_modules/@microsoft/microsoft-graph-client/lib/src/GraphError.js:34:28)
    at GraphErrorHandler.constructErrorFromResponse ...

Configuration

No response

Other information

Symptoms of the problem
Subscription fails to create for some users. This has repeatedly happened for the same user in the same organization (see error message). I have confirmed with our customer that there is nothing unique about that users' Microsoft Outlook account, and other users in their organization are able to use the Outlook subscription feature of our app just fine.

When did the problem begin?
Tue, Feb 4, 2025, 3:02 PM (UTC-08:00) Pacific Time (US & Canada)

Please provide additional details
We have many users using our Outlook subscription feature. The API has been quite buggy. The error here is the most severe and isn't not possible to overcome as far as we're aware. Additionally, the subscription endpoint sometimes hangs indefinitely and results in a 'request cancelled' error. This requires us to manually retry the creation of the subscription.