Ajax sometimes fails to parse the large JSON response - php

Question

I am calling a PHP service with below ajax code :
jQuery.ajax({
url: 'index.php?option=com_sheet&task=getReportsData',
timeout: 300000,
dataType: "json",
type: "GET",
data: {
'project': projectsStr,
'startweek': startweek,
'endweek': endweek
}
}).done(function(response) {
if (response.success && response.data) {
var projectData = response.data;
if (projectData.projectReportData.length > 0) {
loadDataTable(response.data);
} else {
jQuery('#projectReportData').html("<br/><h2>No Matching Results</h2>");
}
}
}).fail(function(jqXHR, textStatus) {
jQuery(".overlay")[0].style.display = '';
if (textStatus === 'timeout') {
alert('Failed from timeout');
}
alert(textStatus);
});
The php code that provides the response prints the response with :
$data = (object)array_merge(['projectReportData' => $projectReportData]);
header("Content-Type: application/json");
$post_data = json_encode($data, JSON_FORCE_OBJECT);
ob_start('ob_gzhandler');
echo new JResponseJson($data);
ob_end_flush();
jexit();
This ajax works fine sometimes and is able to parse the JSON. But most of the times gives "parsererror".
Below code fetch the data from the DB :
$db = JFactory::getDbo();
$query = "CALL getReportData('" . $startweek . "','" . $endweek . "')";
$db->setQuery($query);
$res = $db->query($query);
$returnArr = [];
$i = 0;
while( $r = $res->fetch_assoc()){
$projectReportData[$i] = $r;
$i++;
}
when i change dataType to text the pointer reaches the .done() but console.log(response) returns a blank.