Vimeo API not responding to "If-Modified-Since" header

According to Vimeo API documentation:

The If-Modified-Since header enables you to return only those API resources that have been modified since a particular date and time.

The header looks like this: If-Modified-Since: {ddd}, {D} {MMM} {YYYY} {HH}:{mm}:{ss} {Z} NOTE: If your formatting codes are rusty, Tue, 20 Jun 2023 14:42:36 GMT is an example.

If none of the resources have been modified since this date, the API returns an empty response body and the HTTP status 304 Not Modified.

I'm using the Official PHP library for the Vimeo API.
According to GitHub issue #130, the PHP library's request() method accepts an array of headers. And this commit shows how the $headers array is passed and parsed:

public function request($url, $params = array(), $method = 'GET',
                        $json_body = true, array $headers = array())

// Set the headers
foreach ($headers as $key => $value) {
  $curl_opts[CURLOPT_HTTPHEADER][] = sprintf('%s: %s', $key, $value);
}

But when I pass the future date shown in the example, I still receive a full list of videos rather than the "empty response body and the HTTP status 304 Not Modified" specified in the documentation.

What am I doing wrong?

$fields = array(
  'created_time',
  'modified_time'
);

$params = array(
  'page' => $page,
  'filter' => 'embeddable',
  'filter_embeddable' => true,
  'fields' => implode(',',$fields)
);

$headers = array(
  'If-Modified-Since' => 'Tue, 20 Jun 2023 14:42:36 GMT'
);

$json_body = true;
$method = 'GET';

$response = $vimeo->request('/me/videos', $params, $method, $json_body, $headers);

Result:

Array
(
    [0] => Array
        (
            [created_time] => 2018-06-05T19:27:18+00:00
            [modified_time] => 2018-06-29T19:12:21+00:00
        )

    [1] => Array
        (
            [created_time] => 2016-06-02T03:01:40+00:00
            [modified_time] => 2019-04-30T06:15:29+00:00
        )

    [2] => Array
        (
            [created_time] => 2016-05-29T05:31:46+00:00
            [modified_time] => 2019-04-25T07:46:53+00:00
        )

    ....

---

Edit

Based on this answer (not about Vimeo), it seems that the API might return the entire set of videos if even one of them was modified after the "If-Modified-Since" date.

If anything has changed in the entire response, then it will send the entire response to you.

But I'd still expect the result to be empty if the date is in the future. Am I misunderstanding?

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