The site at https://foe.org/ is hosted by WP Engine. WP Engine blocks requests from user-agents that they are not familiar with. This prevents Discourse oneboxes from displaying for many WP Engine hosted sites.
This has come up a few times in the past. The way to get it resolved is for the owners of the WP Engine hosted site to contact WP Engine and ask them to allow the Discourse Forum Onebox User Agent. If you are an owner of https://foe.org/ it would be great if you could contact WP Engine and ask them to do this.