Why asp.net randomly create cookies on some devices?

I have a Web Server:

Windows Server 2008 R2 X64

IIS version : 7.5.7600.16385

(By this link We checked) installed .net on our machine is: 4.7 (460805)

Our website has been developed on .net 4.7 and MVC 5 and we use ASP.NET Forms Authentication to authenticate our clients.

On the loading of login page, we write a simple test cookie:

 HttpCookie cookie = new HttpCookie("test", "123"); 
 cookie.Expires = DateTime.UtcNow.AddYears(1);
 Response.Cookies.Add(cookie);

when user click on the login button we read that cookie and count all cookies:

  if (Request.Cookies["test"] == null)
        {
           if (Request.Cookies.Count > 0)
                {
                    for(int i=0;i< Request.Cookies.Count;i++)
                        Loger.PointLoger.LogIt("Cookies:" + 
                        Request.Cookies[i].Name 
                       +"   Value: "+ Request.Cookies[i].Value);
                }

              HttpBrowserCapabilitiesBase bc = Request.Browser;
         ...//Some codes to log 
      }

By this way we can be sure, writing cookies are enable and we can write our authentication cookie. If we can not read that test cookie we redirect user to another page and ....

Some our users reported, they can not login to our website and our server logs shows that we can not write cookie on their devices , the important thing is writing cookies are enable on their devices.

After doing some testes we found that writing cookies are randomly or something like that and one time its OK and another time its not.

By this codes we collected some information of out users:

   HttpBrowserCapabilitiesBase bc = Request.Browser;
                ...
        "IsMobileDevice:" + bc.IsMobileDevice
         "-Browser:" + bc.Browser
                ...

For example :

Cookies.Count: 2

Cookies:_ga   Value: GA1.2.163980100.1507000247
Cookies:_gid   Value: GA1.2.1373100693.1518900032

 IsMobileDevice:True
-Browser:Chrome
-Beta:False
-Platform:Unknown
-Type:Chrome47
-Version:47.0
-MobileDeviceModel:Unknown
-MobileDeviceManufacturer:Unknown
-GatewayMajorVersion:0
-MinorVersion:0
-MinorVersionString:0
-MajorVersion:47
-GatewayVersion:None
-Id:chrome
-HasBackButton:True
-Cookies:True
-ClrVersion: 0.0
-InputType:keyboard

Whey these cookies ( _gid and _ga) have been written but our test cookie not?

I read some posts like :

Asp.Net Forms Authentication when using iPhone UIWebView

ASP MVC Cookies not persisting

C# Login code not work on safari

Strange problem with cookies in Safari and Asp.net

Now the question is that: IS THAT A BUG ON .NET 7 ? IF YES WHAT IS THE SOLUTION?

I past here some codes of our project and because of security I replaced some codes with "....." :

On Web.config:

<authentication mode="Forms">
  <forms domain=".mysite.com" name="abc"  cookieless="UseCookies"
    enableCrossAppRedirects="true" loginUrl="/Accounts/Login" 
    timeout="2880" requireSSL="false" path="/" />
   </authentication>

 <machineKey compatibilityMode="Framework45" validationKey="C121487......" 
 decryptionKey="7E43716E4C97....." validation="SHA1" decryption="AES" />

    <sessionState mode="InProc" customProvider="DefaultSessionProvider" cookieless="UseCookies"  cookieName="debnf">
      <providers>
        <add name="DefaultSessionProvider" type="System.Web.Providers.DefaultSessionStateProvider, System.Web.Providers, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf38...." connectionStringName="DefaultConnection" />
      </providers>
    </sessionState>

In App_Browsers :

 <browsers>
    <browser refID="Default">
      <capabilities>
      <capability name="cookies" value="true" />
      </capabilities>
   </browser>
 </browsers>

UPDATE:

This is my main codes :

I write this test cookie here 

public ActionResult Login()
{     
  HttpCookie cookie = new HttpCookie("test", "123");
  cookie.Expires = DateTime.UtcNow.AddHours(1);
   Response.Cookies.Add(cookie);
  return View();
 }

and when user click login I check that cookie here:

 [HttpPost]
 public ActionResult Login(LoginModel loginModel)
  {
    if (Request.Cookies["test"] == null)
      {
     .....

      } 

  }