Vhosts ignored depending on request origin

CentOS 6.8, Plesk 12.5.30, Apache 2.2.15, no Nginx.

I have a server configured with multiple domains, everything created through the Plesk interface following the relevant documentation and otherwise there has been no hanky-panky or hand-editing of configs. Everything was working great until a recent `yum upgrade` (unfortunately rollback is not an option) and update to the most recent version of Plesk.

Now, depending on the origin of the request, the server either works totally fine...

Vhosts ignored depending on request origin