I noticed this forum opens youtube and I assume all other links in the current browser tab by default which overrides my default browser settings, but I did find forum preference to turn it off. Just mentioning it in case anyone is puzzled about site’s behaviour. I did not think you could do that, as it is rare that a site overrides my default browser preferences - just sayin.
The behaviour I think you’re experiencing is actually a feature. It is using the discourse server to retrieve the site information (on the server, not in your browser) and then providing am image representation of that to your client so that it can know what it would see BEFORE retrieving the content. This is a safety feature, not a bug.