Insecure Login Page for Subdomain Sites (SSL)

I have a demo site - multisite setup - where I'm trying to force SSL on login / admin pages. I've installed the SSL certificate and added the following line to the wp-config file:

define('FORCE_SSL_ADMIN', true);

My site uses Domain Mapping (Administration and Login mapping set to 'Original Domain'). When I go to the main site's login page, SSL works fine. When I go to a subdomain site's login page, the padlock is broken. (e.g.

Based upon the errors in Chrome Console, it looks like the errors are coming from WP core files and not a plugin, but I'm not sure ... and I'm not sure how to fix this: