Can't access cookies from document.cookie in JS, but browser shows cookies exist

You are most likely dealing with httponly cookies. httponly is a flag you can set on cookies meaning they can not be accessed by JavaScript. This is to prevent malicious scripts stealing cookies with sensitive data or even entire sessions.

So you either have to disable the httponly flag or you need to find another way to get the data to your javascript.

By looking at your code it should be easy to disable the http only flag:

Response.AddHeader("Set-Cookie", "CookieName=CookieValue; path=/;");
Response.SetCookie(new HttpCookie("session-id") { Value = Guid.NewGuid().ToString(), HttpOnly = false });
Response.SetCookie(new HttpCookie("user-name") { Value = data.Login, HttpOnly = false });

Now you should be able to access the cookie information from JavaScript. However I don't know exactly what kind of data you are trying to get so maybe you can go for another approach instead and for example render some data attribute on the page with the information you need instead of trying to read the cookie:

<div id="example" data-info="whatever data you are trying to retrieve"></div>

console.log(document.getElementById('example').getAttribute('data-info'));

I would say http only is your first culprit but this can also occur by not setting the scope of your cookie.

If the site has been redirected from another domain, you will need to look into setting the scope of the cookie. Domain and Path defines the scope of the cookie, which URLs the cookie should be sent to. Depending on this, you might not see the cookie in your response.

I ran across this issue when setting a cookie on a successful SAML SSO login and couldn't retrieve the cookie from the Document because it was never send as part of the request.

keep an eye also to the cookie's Path attribute, as the cookie is only visible to subdirectories under Path. I had your issue and I solved setting Path "/"