SeguirBlog are new shares also like that? Or only some of the links previously published to Facebook?
Usually clearing Flarum's cache should fix it. But Facebook will have cached its previous results so, it won't get fixed immediately on Facebook side. I believe there's a page somewhere on the Facebook developer area that allows re-scanning a link manually.
If you have multiple languages installed, you might have to visit each homepage after cleaning the cache to make sure it has been properly re-filled. There's a possibility the problem appeared if Google was the first visitor of a particular language after the cache clear.
