From the client's description:

Our application uses an ActiveX-Control and normal Qt-Widgets (like QLineEdit, QCheckBox, ...). Furthermore, our application switched off the title bar.
If these three condition are met, no paint event occurs for the normal Qt-Widgets - e.g. you can enter text in QLineEdit but it's not shown.
However, all other events (like KeyDown, .) are recognized correctly. Entering values in QLineEdit are shown if you switch on the titlebar.

To demonstrate the bug I have added a small demo-program.

The client reported on Qt 5.5, but I verified that problem exists also on Qt 5.8.