After running into strange issues with the recent round of Plex updates that Plex has been pushing, it has been determined that Plex has introduced something which causes reverse proxies not to function.
There is nothing we can do to fix this, this code must be fixed by Plex. It is uncertain if the change was intentional or accidental.
With that said, in order to setup Plex, you'll have to use the tried and true SSH tunnel method again. Hopefully the Plex team will fix this and not leave it in the state it's in.