https://fr.wikisource.org/wiki/Cat%C3%A9gorie:Livres_%C3%A0_corriger
This category has 10,494 pages (books), but the list stops at 1997. https://fr.wikisource.org/w/index.php?title=Sp%C3%A9cial:IndexPages&limit=500&offset=1500&key=incategory%3A%22Livres+%C3%A0+corriger%22&order=size&sortascending=1
Even more weird, when forcing the offset, we get https://fr.wikisource.org/w/index.php?title=Sp%C3%A9cial:IndexPages&limit=100&offset=10000&key=incategory%3A%22Livres+%C3%A0+corriger%22&order=size&sortascending=1
Search for incategory:"Livres à corriger" works, so it probably comes from the Proofreadpages extension