EDIT: I mixed up this and another post so this answer is mostly incorrect. Leaving it for context..
The original issue on this post was that Microsoft's generic custom non-gallery SCIM client treats manager as a simple string rather than complex.
String: "manager":"123"
Complex: "manager": { "value":"123" }
The issue that you are highlighting is that aside from the client sending the manager value as the wrong data type, you would like our SCIM client to send the manager's displayName value as well. This isn't something that is on our roadmap, and it will not be added to our roadmap, as the SCIM spec says:
manager
The user's manager. A complex type that optionally allows service
providers to represent organizational hierarchy by referencing the
"id" attribute of another User.
value The "id" of the SCIM resource representing the user's manager. RECOMMENDED.
$ref The URI of the SCIM resource representing the User's manager. RECOMMENDED.
displayName The displayName of the user's manager. This attribute is OPTIONAL, and mutability is "readOnly".
The expectation from both the spec and Microsoft is that the displayName value is optional and only returned by the service provider, not provided by the client. You should already have the manager's displayName attribute present if it has been provisioned separately as another user, and should be able to make calls internal to your application to retrieve that.