Problem with iOS and header API versioning

Question

We have implemented API versioning using headers, so in HttpClient on the mobile app code (Xamarin Forms), we do something like this:

_myHttpClient.DefaultRequestHeaders.Add(CustomRequestHeaders.ApiVersion, "2");

In the backend, we have a UsersController and a Users_v2Controller, and we need to consume the method GetUsersByRole.

UsersController:

[Authorize]
    [RoutePrefix("api/Users")]
    public class UsersController : BaseApiController
    {
        // ...

        [HttpGet]
        [Route("")]
        public IHttpActionResult GetUsersByRole([FromUri] int clientId, [FromUri] string role)
        {
            if (ApiVersion == "2")
                return RedirectToRoute("GetUsersByRole_v2", new { clientId = clientId, role = role });

                return Ok ("V1 results");
        }

        // ...

    }

Users_v2Controller:

[Authorize]
    [RoutePrefix("api/Users")]
    public class Users_v2Controller : BaseApiController
    {
        // ...

        [HttpGet]
        [Route("", Name = "GetUsersByRole_v2")]
        public IHttpActionResult GetUsersByRole([FromUri] int clientId, [FromUri] string role)
        {
                return Ok ("V2 results");
        }

        // ...

    }

Now, I am consuming the GET /api/users?clientId=1&role=Admin with ApiVersion=2 from android, iOS and Postman to double check:

• Android: returns V2 results
• Postman: returns V2 results
• iOS: returns 401: unauthorized but if i set ApiVersion=1 it works and returns V1 results

What could the problem be? Why would iOS say i am unauthorized?