Google my business not fetching reviews correctly

Question

I'm currently using Google's api for fetching reviews of a Google My Business Account. We do this for our clients, which have to enable oauth for us, so we can have an access token and a refresh token.

The response we get, has the following structure: url: https://mybusiness.googleapis.com/v4/accounts/{account_id}/locations/{location_id}/reviews

{
    "reviews": [
        {
            "reviewId": "some-id,
            "reviewer": {
                "profilePhotoUrl": "some-url",
                "displayName": "Some Sample Name"
            },
            "starRating": "FOUR",
            "comment": "Some long comment left by the user.",
            "createTime": "2021-02-15T14:35:19.252Z",
            "updateTime": "2021-02-15T14:35:19.252Z",
            "reviewReply": {
                "comment": "Some long reply by the business.",
                "updateTime": "2021-03-04T16:49:32.973Z"
            },
            "name": "accounts/{account_id}/locations/{location_id}/reviews/{review_id}"
        },
        ...
    ],
    "averageRating": 4.5,
    "totalReviewCount": 1312,
    "nextPageToken": "some-page-token"
}

The problem is, for some set of clients, we are getting just an empty object (literally, we are receiving {} as the response body). We are not sure why this is happening, since we are able to see reviews on the client's web site, and we are not getting any error. We've refreshed tokens and refreshed tokens without any success.

I've tried posting a support ticket with GCP in order to get some guidance, but it seems that you don't get any help in this kind of issues if you don't have the right support plan.

Anyhow, any help on this issue (why is happening? can it be fixed? is it a setup problem? etc.) is very much appreciated.