title: AspNet Core login not working but works in incognito window
body: <p>We have a Asp.net Core 2.0 application with MVC and React frontend that we didn't write but now maintain. It uses the built-in authentication for security. We're testing a major update but authentication wasn't altered. The problem is our QA site logins redirect back to the login page with no error for some but not all users. (This is in Chrome though Edge has worked so far.) Incognito window and localhost both work fine and login correctly. Production also currently works but we need to ascertain this won't happen when we deploy the update there.</p>
  <p>The site runs on a subdomain (think <em>my.appsite.com</em>) so the QA site is <em>qa.my.appsite.com</em>, if that might make a difference. Localhost and QA are http, production is https.</p>
  <p>One thing we've noticed is there's no <em>.AspNetCore.Cookies</em> cookie on QA, but there is in incognito plus localhost and production. We've tried the various values for <code>options.Cookie.SameSite</code> and <code>options.Cookie.SecurePolicy</code> with no change in behavior.</p>
  <p>The fact it works in an incognito window suggests a caching issue, but we've tried clearing cookies, and clearing the site cache. No luck. Clearing all browser cache and cookies did work, but that's obviously not a viable solution.</p>
  <p>I know this is extremely vague, but I don't know what I don't know, so I'm at my wits' end. If you need code samples or other information, I'll be happy to oblige.</p>