The OpenAPI Specification for the List all client role assignments (oauth2/v1/clients/{clientId}/roles) has an incorrect response type defined. It should be Array [ One of: Standard Role Assignment / Custom role assignment ] instead of just One of: Standard Role Assignment / Custom role assignment. Notice the missing Array.
Compare it with the correct definition of the List all group role assignments (api/v1/groups/{groupId}/roles) and List all user role assignments (api/v1/users/{userId}/roles).
This affects the generated code, at least the Okta .NET management SDK, but other SDKs might be affected as well.