I assume that it somehow scrapes the page for some sort of header text, but this doesn’t seem to always work.
For example, titles are suggested for spacenews.com articles, but not for europeanspaceflight.com articles. What differences result in suggested titles for one but not the other?
I don’t have any specific technical knowledge of how Lemmy-UI does things, but I’m reasonably confident that it uses the og:title. This is a tag in the site’s metadata designed specifically to be the title that displays in places like Facebook and Twitter auto-generated thumbnails, and which is also used in things like Lemmy and Reddit which provide a “suggested title” to users.
Ah, so the problem is with how some sites configure (or fail to configure) their metadata, rather than a problem with Lemmy?
Could be. Though Lemmy could also use the title property (the one users see in their tab header) as a fallback option if it wanted to, or some other fallback.