Assign app role to newly created/invited user results in 400 BadRequest

Describe the bug

I am successfully creating an invite to a b2b guest user using Invitations.PostAsync, I'm then trying to set an app role using AppRoleAssignedTo.PostAsync on that user but it fails with statusCode 400, errorCode 'Request_BadRequest' and errorMessage 'Not a valid reference update'.

I'm even able to query the newly created user with Users[userObjectId].GetAsync prior to trying to set role.

Trying to set the role on that user a few minutes later works.

Expected behavior

Being able to set user app role on the newly created invited user? Or get a more explaning message (retry-after or similiar)? Something more actionable, there's nothing wrong with the request (it's not a bad request).

How to reproduce

// request bodies removed for brevity 
var userObjectId= await _client.Invitations.PostAsync(requestBody);
var result = await _client.Users[userObjectId].GetAsync();
var roleAssignment = await _client.ServicePrincipals[Environment.GetEnvironmentVariable("EntraEAObjectId")].AppRoleAssignedTo.PostAsync(roleRequestBody);

SDK Version

5.53

Latest version known to work for scenario above?

No response

Known Workarounds

No response

Debug output

</details>


### Configuration

_No response_

### Other information

_No response_

Start by reproducing the sequence using Invitations.PostAsync, Users[userObjectId].GetAsync, and AppRoleAssignedTo.PostAsync, then compare the immediate 400 response with the later successful call. Done means the SDK or service scope is confirmed and the behavior has a targeted fix, test, or actionable retry guidance.