Elements with DropShadow are invisible on Windows 7 and Intel 4 Series Integrated Graphics. The Intel driver is installed and Qt is using OpenGL.
Forcing ANGLE using QT_OPENGL=angle works.
I've also found a Stack Overflow question with the exact same problem by someone else: https://stackoverflow.com/questions/47378457/qml-qtgraphicaleffect-are-not-rendered-in-target-machine
https://doc.qt.io/qt-5/windows-requirements.html mentions a built-in QT_OPENGL_BUGLIST, maybe Intel Series 4 GPUs (and earlier, but I think they don't support OpenGL 2.1 anyway) can be added there?