You see what you see when you paste the site into Iframely URL Debugger - Open Graph, Twitter Cards, oEmbed
Then you ask the site why they don’t follow the standards that make their site usable.
If required you pay to have a custom plugin developed or become a customer of Discourse.org as customer needs are taken care of first.
Is there a reason not to share the site name here? There is no way to diagnose the problem without being able to see it. What if you paste a link here or in try what happens?