Well, Fandom came up with the new UI Design tonight... For me, it's... fine. But might as well, the admin had to change to look of this page like in old days, but with more advencing, since this update came out.
@Martim4820 it had started a long ago this year. I first noticed it when The After Dark Scene thumbnail changed from the cover art to the car crash animation. Then it was like two or three weeks ago when it began to affect other pages.
A quick update, according to the Fandom staff reply, the problem is probably caused by caching. They had edited TADS page to try and regenerate some data. I don't know how exactly the mechanism works, but it actually worked. TADS thumbnail now reverts back to the cover art.
UPDATE #1: The Fandom Team has now resolved the issue with the category page where the very first image on any page, including the placeholder, was not displayed. However, for some reason, they removed the subcategory placement, which could be useful, but hey, it's a decent try for them all. Thank all of you that, Fandom!