Cookie not deleted after logout with Asp.Net 5 Identity 3.0

The problem is that your RedirectToAction overwrites the redirect to the Identity Server endsession URL that SignOutAsync issues.

(The same explanation for the same problem is given here by Microsoft's HaoK.)

Edit: The solution is to send a redirect URL in an AuthenticationProperties object with the final SignOutAsync:

// in some controller/handler, notice the "bare" Task return value
public async Task LogoutAction()
{
    // SomeOtherPage is where we redirect to after signout
    await MyCustomSignOut("/SomeOtherPage");
}

// probably in some utility service
public async Task MyCustomSignOut(string redirectUri)
{
    // inject IHttpContextAccessor to get "context"
    await context.SignOutAsync("Cookies");
    var prop = new AuthenticationProperties()
    {
        RedirectUri = redirectUri
    });
    // after signout this will redirect to your provided target
    await context.SignOutAsync("oidc", prop);
}