@da3v When I just visited that page (10 minutes after you posted) the lesson 12 thumbnail looks just fine. For me the thumbnail with the “4D 3D 6- 6-“ third line is the one for lesson 13 ‘Latin Minor Blues’.
Ah, my mistake. On iOS, the thumbnails huddle a little bit closer to the “Go to Lesson” link for the lesson above. At first glance, the shorter 8 bar blues thumbnail looked like it went with the Latin minor blues, so it seemed like the next thumbnail up went with lesson 12.
(The thumbnails definitely do take you to the correct lesson.)