I just wanted to mention that I solved this issue by editing /etc/foreman/plugins/katello.yaml and adding our proxy to the section :cdn_proxy:.
I then added the http_proxy and https_proxy environment variables to:
- /etc/sysconfig/dynflowd
- /etc/sysconfig/foreman
- /etc/sysconfig/httpd
The issue I experienced appears to have been resolved in new(er) versions of Foreman and Katello.