QFileSystemWatcher does not monitor a file

Problem described here: https://forum.qt.io/post/716353

If I edit file in an editor - QFileSystemWatcher triggers `fileChanged()` signal. If I do:

    std::ofstream output_file(file_Name, std::ios_base::out | std::ios_base::trunc);
    output_file << content;
    output_file.flush();

Or analogous with Qt classes (QFile and QTextStream), then QFileSystemWatcher does not work, no signal. QFileSystemWatcher::files() shows files is still being monitored after truncation.

File system is ext4. Minimal working example: https://forum.qt.io/post/716370 and another one: https://forum.qt.io/post/716365