Pro sites users cannot login due to insecure endpoints

Mixed Content: The page at wp-login.php was loaded over a secure connection, but contains a form which targets an insecure endpoint wp-login.php. This endpoint should be made available over a secure connection.

I am having a similar issue. Every URL for my subdomain example.removed.com is https except for form post url

<form name="loginform" id="loginform" action="http://example.removed.com/wp-login.php" method="post">

The parent site "removed.com" has a wildcard SSL so every subdomain has SSL. The front end and back end (login screen) are reachable via SSL, but when I try to actually login using a user/pass I know to be correct, the screen just redirects back to wp-login.php. I am assuming it is because of the console error:

Mixed Content: The page at wp-login.php was loaded over a secure connection, but contains a form which targets an insecure endpoint wp-login.php. This endpoint should be made available over a secure connection.

I confirmed the blog (example.removed.com) settings have both Siteurl and Home using https:// but it does not change the insecure endpoint wp-login.php

When I toggle off:

Domain Mapping
Version 4.4.2.4 | By WPMU DEV

No more "insecure endpoint wp-login.php" console error and I am able to login. So I guess my question is : How do I have a client who is able to login using a subdomain of the parent site while others use their custom domain name mapped to their login screen?

Thanks