Browser (Chrome, Firefox) cancels download stream prematurely, after 5 seconds

I have a function that is streaming a csv file for the client to download. I find that after 5 seconds, the browser will terminate the connection. I have used postman and curl, and it completes the output so I can eliminate that the buffer is full / not outputting.

On firefox I get the reason:

NS_ERROR_NET_PARTIAL_TRANSFER

    public function random_stream()
    {
        ini_set('zlib.output_compression', 'Off');

        // Clear output buffering
        while (ob_get_level()) {
            ob_end_clean();
        }
        // Set headers first
        header('Content-Type: text/csv');
        header('Content-Disposition: attachment; filename="test.csv"');
        header('Pragma: no-cache');
        header('Expires: 0');

        // Now open output stream
        $file = fopen('php://output', 'w');

        // Optional: output a CSV header row
        fputcsv($file, ['Column 1', 'Column 2', 'Column 3']);

        $start_time = time();
        while ((time() - $start_time) < 10) {
            $row = [
                $this->randomString(),
                $this->randomString(),
                $this->randomString()
            ];
            fputcsv($file, $row);
            flush(); // optional but ensures streaming output
        }

        fclose($file);
        exit;
    }


    function randomString($length = 8)
    {
        $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $charactersLength = strlen($characters);
        $randomStr = '';
        for ($i = 0; $i < $length; $i++) {
            $randomStr .= $characters[rand(0, $charactersLength - 1)];
        }
        return $randomStr;
    }

Am I setting the headers wrong or not flushing properly for the browser to keep alive?

response headers:

check what is wrong with this response:

GET /random_stream HTTP/1.1
Host: ***obscured***
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:140.0) Gecko/20100101 Firefox/140.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br, zstd
Connection: keep-alive
Upgrade-Insecure-Requests: 1
Sec-Fetch-Dest: document
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: none
Sec-Fetch-User: ?1
Priority: u=0, i
Pragma: no-cache
Cache-Control: no-cache

HTTP/1.1 200 OK
Transfer-Encoding: chunked
Cache-Control: no-cache
Connection: Keep-Alive
Content-Disposition: attachment; filename="test.csv"
Content-Type: text/csv;charset=UTF-8
Date: Wed, 06 Aug 2025 16:56:01 GMT
Expires: 0
Keep-Alive: timeout=5, max=100
Pragma: no-cache
Server: Apache/2.4.41 (Ubuntu)