The same issue happens with the simple search form: https://fr.wikisource.org/w/index.php?search=incategory%3AInde+OR+incategory%3ABouddhisme&title=Sp%C3%A9cial%3ARecherche&go=Lire

So I think it is related to the search engine and not to ProofreadPage

Moving All-and-every-Wikisource and Possible-Tech-Projects here from unnecessary parent task.

This looks more like a "bug". Is it a suitable candidate for Possible-Tech-Projects ?

Ok, I understand now that its parent was a tracker, commenting there now. Ignore previous comment.

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!

From https://www.mediawiki.org/wiki/Help:CirrusSearch/Logical_operators

AND and OR do not interact predictably with special keywords (like insource: or hastemplate:) or with namespaces (like Talk: or User:) and probably should not be used in conjunction with either.

So I think we can close this as a duplicate of the upstream search platform issue: T164589

Also note that the following syntax does work:

incategory:"Inde|Bouddhisme"

https://fr.wikisource.org/wiki/Sp%C3%A9cial:IndexPages?key=incategory%3A%22Inde%7CBouddhisme%22&order=quality&limit=250