Google drive API thumbnail not displayed - google-apps-script

Question

I can not get all thumbnails using Google Drive API. They are randomly displayed.
Here is the code I use:
<script src="https://code.jquery.com/jquery-1.11.3.min.js"></script>
<script type="text/javascript">
function gapiLoaded() {
gapi.load('client', intializeGapiClient);
}
async function intializeGapiClient() {
await gapi.client.setToken({access_token:'ya29.a0AVvZVs...'});
loadDriveApi();
};
function loadDriveApi() {
gapi.client.load('drive', 'v3', listFiles);
}
function listFiles() {
var parentID = 'root';
var request = gapi.client.drive.files.list({
'q': `'${parentID}' in parents and trashed = false`,
'fields': "nextPageToken, files(id,thumbnailLink,size,webViewLink,parents,shared,name,createdTime,mimeType,properties)",
});
_files = [];
i = 0;
request.execute(function(resp) {
var files = resp.files;
if (files && files.length > 0) {
for (var i = 0; i < files.length; i++) {
var file = files[i];
_files += file.id+'|'+file.name+'|'+file.size+'|'+file.thumbnailLink+'|'+file.mimeType+'|'+file.webViewLink+'|'+file.parents+'|'+file.createdTime+'\r\n';
if (i+1 == files.length){
console.log(_files);
}
}
}
});
}
</script>
<script async defer src="https://apis.google.com/js/api.js" onload="gapiLoaded();"></script>
and here is animated gif displaying the result: https://i.postimg.cc/26FMy6Hs/g1.gif
How to get all thumbnails, please?