Blank windows when reuse instance of QDialog::exec()

Starting from Qt 5.12.4 to reuse instance of QDialog creates blank window, content is shown to user only after manualy resizing window by user ( imposibble with fixed size dialog )

Bug in Windows 10 is first introduced in 5.12.4, Previous versions (<5.12.4) work as intended.

Code to replicate bad behaviour:

// Prepare test QDialog
auto dialog = new QDialog( this );
auto layout = new QVBoxLayout();
layout->addWidget(new QLabel("This text should be visible") );
dialog->setLayout( layout );

// First use of dialog
dialog->exec();
// Correct content is shown

// Second use of dialog
dialog->exec();
// Empty window is shown - it is visible again on size change by user.

First use of dialog->exec() produces:

Second use of dialog->exec() produces: