Cannot set QML flat style

I've tried to use the Flat style on WinRT without any success, the application prints this message and the style is not applied:
qt.winrtrunner.app: WARNING: Cannot find style "Flat" - fallback: ":/QtQuick/Controls/Styles/Base"

The same code works correclty with mingw and android packages of 5.5.0 RC.

The issue can be reproduced creating a new project, the only changes I made to add the Flat style are:

• add in main.cpp
  qputenv("QT_QUICK_CONTROLS_STYLE", "Flat");
  just before
  engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
• add in main.qml
  import QtQuick.Controls.Styles.Flat 1.0
  import QtQuick.Extras 1.0

I might have missed some step to set correctly the flat style, but I find strange that it does not work on WinRT only