Note that speed dials on the start page are bookmarks too, and Opera GX comes with some default speed dials that have special Opera GX thumbnails. For those sites, since they're already bookmarked, if you want them on the bookmarks bar, you'll actually be moving them to the bookmarks bar. But, the problem is, those special speed dial thumbnails aren't supported on the bookmarks bar.
To fix it, goto the URL opera://bookmarks. Move the bookmark to the bookmarks trash. If the site is still present in the speed dial bookmarks folder too, move that bookmark to the trash too. Then, empty the bookmarks trash. Then, goto the site and click the heart icon to bookmark it to the bookmarks bar and the favicon will appear as you expect.
