If we compare the JSON response of these two requests to /mentees we can see how the mentees property is an Array whereas when the query parameter offset is higher than zero the returned structure is an array-like Object.

• https://en.wikipedia.beta.wmflabs.org/w/rest.php/growthexperiments/v0/mentees?order=desc&sortby=last_active&limit=1&offset=0

{
    "mentees": [
        {
            "username": "redacted_1",
            "user_id": "1"
            ...
        }
    ],
    "totalRows": 2,
    "limit": 1,
    "offset": 0
}

• https://en.wikipedia.beta.wmflabs.org/w/rest.php/growthexperiments/v0/mentees?order=desc&sortby=last_active&limit=1&offset=1

{
    "mentees": {
        "1": {
            "username": "redacted_2",
            "user_id": "2"
        }
    },
    "totalRows": 2,
    "limit": 1,
    "offset": 1
}

This is not ideal from an API output consistency point of view. It also leads API clients to have to treat differently similar responses.