Facebook requires thumbnails defined with og:image to be minimally 200px by 200px and recommends a size of 1200x630. OpenGraphMeta generates one that is 300x100, triggering the following error in the sharing debugger:
Provided og:image URL, https://url/images/thumb/c/cb/Diagramme_de_coeur.png/99px-Diagramme_de_coeur.png was not valid because it did not meet the minimum size constraint of 200px by 200px.
In OpenGraphMeta.class.php, changing line 76 to:
$meta['og:image'] = wfExpandUrl( $mainImage->createThumb( 1200, 630 ) );
fixes the issue.