cache problem with /caslogin/?returnto= query parameter

Overview

When building a dynamic login link that uses the current page path in the following format /caslogin?returnto=<CURRENT-PAGE-PATH>, the first login will return you to the proper page, but if you log out, go to another page, and log back in, you will not be returned to the correct page. Instead, you will be returned to the page that your first login returned you to. Clearing the cache will resolve the problem for one iteration. In short, some caching bug is making the returnto parameters only work successfully for the first login in a cache cycle.

Dynamic Login Twig code

{% if logged_in %}
    <a href="/user/logout">Log Out</a>
  {% else %}
    <a href="/caslogin{% if not is_front %}?returnto={{ path('<current>') }}{% endif %}">Log In</a>
  {% endif %}

Steps leading to bug

1. clear all site cache
2. go to /some/page/x
3. click Log In link, which points to http://<MY-SITE>/caslogin?returnto=/some/page/x
4. You are successfully logged in and returned to /some/page/x
5. Click Log Out link, which return you to the home page
6. go to /some/page/y
7. click Log In link, which points to http://<MY-SITE>/caslogin?returnto=/some/page/y
8. You are authenticated, but inaccurately returned to /some/page/x
9. You can clear all site cache and the first login redirect will return you to the correct page, but subsequent login redirects won't