Error opening file for writing on Android

I have successfully deployed files to Android into

assets:/qml/*

folder. Added android.permission.READ_EXTERNAL_STORAGE and android.permission.WRITE_EXTERNAL_STORAGE permissions. No problem to open file for reading there: 

 if (source.open(QIODevice::ReadOnly | QIODevice::Text)) { ... }

However have problem for writing - construction:

if (dest.open(QIODevice::WriteOnly | QIODevice::Truncate | QIODevice::Text)) { 
    ... 
} else {
    qDebug() << destPath << ": " << dest.errorString();
}

returns "No such file or directory" if file does not exist (on Desktop it creates new one) and if I deploy empty file to open and rewrite it (copy source file to dest) it returns "Unknown error" and does not open the file for writing. Regression? Seems something similar was in 5.10: QTBUG-64103?