@Yann for the second paragraph it may be worth describing what you are seeing as the url won't be sustainable once an issue is fixed, and for the sake of completeness of the bug a description or images of the weirdness would be more valuable.

OK.
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
Here we can view the 100 previous books in the list while clicking on "previous 100/100 précédentes" which leads to https://fr.wikisource.org/w/index.php?title=Sp%C3%A9cial:IndexPages&limit=100&offset=9900&key=incategory%3A%22Livres+%C3%A0+corriger%22&order=size&sortascending=1
but then we can't go back to the previous page while clicking on "next 100/100 suivantes".

Here is the same issue on the English Wikisource:
https://en.wikisource.org/w/index.php?title=Special:IndexPages&limit=100&offset=5000&key=incategory%3A%22Index+Not-Proofread%22&order=size&sortascending=1
Clicking on "previous 100" leads to https://en.wikisource.org/w/index.php?title=Special:IndexPages&limit=100&offset=4900&key=incategory%3A%22Index+Not-Proofread%22&order=size&sortascending=1
but we can't go back to the previous list while clicking on "next 100".

the same on pl ws; the problem occurs only when $this->searchTerm is set.
sorry, not only - without searchTerm problem https://fr.wikisource.org/w/index.php?title=Sp%C3%A9cial:IndexPages&limit=100&offset=10000&order=size&sortascending=1 also occurs

Z.

Moving Possible-Tech-Projects here from unnecessary parent task.

Removing the Possible-Tech-Projects tag as we are planning to kill it soon! This project does not seem to fit in the Outreach-Programs-Projects category in its current state, so I am not adding that tag right now!