I dug in and changed the Python library to fix this for now, hoping it’s resolved in a later update or CloudFlare / CloudFront fix the headers.
For anyone else having this problem on Ubuntu 18.04, edit /usr/lib/plexmediaserver/Resources/Python/lib/python2.7/httplib.py
Edit removed by @sa2000 on 21 December 2019
If anyone applied this edit, please uninstall Plex Media Server and download and re-install Plex Media Server
