I am guessing you are using "Page previews": https://www.mediawiki.org/wiki/Page_Previews
(This has to be enabled by each user on https://fr.wikipedia.org/wiki/Sp%C3%A9cial:Pr%C3%A9f%C3%A9rences under "Aperçus de page".)
The tool loads its data from here:
https://fr.wikipedia.org/api/rest_v1/page/summary/Pince_%C3%A0_avoyer https://fr.wikipedia.org/api/rest_v1/page/summary/Pince_universelle
The first of these pages has a "thumbnail" defined, the second doesn't. Unfortunately I don't know what generates this data, but perhaps someone else can help.