Files downloaded with donwloadManager disappears

Question

I have an app that downloads about 6k photos from a server and store them in a folder configured y my app settings, I keep the files hidden from the gallery with .nomedia file and they are only visible in my app gallery, but when I leave my device charging about 5000 photos disappears, and there are only about 920 left in the folder, I totally don't know why files are being deleted.

here is my download file code

    DownloadManager downloadManager = (DownloadManager) activity.getSystemService(Context.DOWNLOAD_SERVICE);
    DownloadManager.Request request;
    fileURL = convertUri(fileURL);
    if (!URLUtil.isValidUrl(fileURL)) { return false; }
    Uri downloadUri = Uri.parse(fileURL);
    String fileName = URLUtil.guessFileName(fileURL, null, MimeTypeMap.getFileExtensionFromUrl(fileURL));
    deleteFileIfExists( new File( getAbsolutePath(path), fileName ));
    request = new DownloadManager.Request(downloadUri);
    request.setAllowedNetworkTypes( DownloadManager.Request.NETWORK_WIFI )
            .setTitle(fileName)
            .setAllowedOverRoaming(false)
            .setVisibleInDownloadsUi(false)
            .setDestinationInExternalPublicDir( getPath(path), fileName );
    downloadManager.enqueue(request);

Answer 1

This weekend i tested with 3 different devices and 2 different download methods, in the one with download manager the files dissapeared, so if someone have the same problem here's how I am donwloading the files:

public static boolean downloadFile(Activity activity, String fileURL) {
    fileURL = convertUri(fileURL);
    if (!URLUtil.isValidUrl(fileURL)) { return false; }

    try {
        String fileName = URLUtil.guessFileName(fileURL, null, MimeTypeMap.getFileExtensionFromUrl(fileURL));
        deleteFileIfExists( new File( activity.getExternalFilesDir(""), fileName ));

        URL u = new URL(fileURL);
        URLConnection conn = u.openConnection();
        int contentLength = conn.getContentLength();

        DataInputStream stream = new DataInputStream(u.openStream());

        byte[] buffer = new byte[contentLength];
        stream.readFully(buffer);
        stream.close();

        File file = new File(activity.getExternalFilesDir(""), fileName);
        DataOutputStream fos = new DataOutputStream(new FileOutputStream(file));
        fos.write(buffer);
        fos.flush();
        fos.close();
    } catch(FileNotFoundException e) {
        return false; // swallow a 404
    } catch (IOException e) {
        return false; // swallow a 404
    }
    return true;
}