Each of the following requests return correct values:

• https://bugzilla.mozilla.org/rest/user?ids=697005&include_fields=last_activity_time,id

{
    "faults": [],
    "users": [
        {
            "id": 697005,
            "last_activity_time": "2022-09-07T00:47:22Z"
        }
    ]
}

• https://bugzilla.mozilla.org/rest/user?ids=159069&include_fields=last_activity_time,id

{
    "faults": [],
    "users": [
        {
            "id": 159069,
            "last_activity_time": "2022-09-04T19:17:00Z"
        }
    ]
}

However, when we request the data for more than one user in the same request, all users will have the same value in last_activity_time field:

• https://bugzilla.mozilla.org/rest/user?ids=697005&ids=159069&include_fields=last_activity_time,id

{
    "users": [
        {
            "id": 159069,
            "last_activity_time": "2022-09-04T19:17:00Z"
        },
        {
            "last_activity_time": "2022-09-04T19:17:00Z",
            "id": 697005
        }
    ],
    "faults": []
}

Merged
https://github.com/mozilla-bteam/bmo/commit/77523932aa28c97d61634d24b6337d151b7f8c43