This might be needed by @Tsevener

Change #1214136 had a related patch set uploaded (by Aude; author: Aude):

[mediawiki/extensions/ReadingLists@master] POC - Make rlelists param optional in readinglistentries query api

https://gerrit.wikimedia.org/r/1214136

Change #1214136 merged by jenkins-bot:

[mediawiki/extensions/ReadingLists@master] Make rlelists param optional in readinglistentries query api

https://gerrit.wikimedia.org/r/1214136

https://en.wikipedia.org/wiki/Special:ApiSandbox#action=query&format=json&list=readinglistentries&formatversion=2 does show all lists:

e.g.

{
    "batchcomplete": true,
    "query": {
        "readinglistentries": [
            {
                "id": 95830209,
                "listId": 5525729,
                "project": "https://en.wikipedia.org",
                "title": "Gary_Evans_(psychologist)",
                "created": "2025-12-19T23:15:30Z",
                "updated": "2025-12-19T23:15:30Z"
            },
            {
                "id": 95830204,
                "listId": 5525729,
                "project": "https://en.wikipedia.org",
                "title": "Robert_Dick_(geologist)",
                "created": "2025-12-19T23:15:21Z",
                "updated": "2025-12-19T23:15:21Z"
            }
        ]
    }
}

Note:
@Jdlrobson-WMF - If the scope of this task is to add API endpoint, then the task is done. I still see the issue - the default list doesn't show all added articles on iOS app "All articles" tab for the use case described in T406532.

I checked the straightforward case

• an article is added on iOS app to a specific list
• the added article will be displayed in "All articles"

On the screenshot below the Reading list "Dec 22 Test 2" displays all articles belonging to that reading list: