To rule out environment and driver problems, we have written three corresponding applications using xlib only, xcb only, and a xlib xcb mix. All three of them work on the same Nvidia graphics card in the same environment.
The attached xlib.cpp, xcb.cpp, xcb_xlib.cpp and qt.cpp can be built following the instructions in build_and_run.sh (replace the QT path in the script to where your Qt installation is first).
Replacing EGL_PLATFORM_XCB_KHR with EGL_PLATFORM_X11_KHR in xcb_xlib.cpp, and using xlib display, causes eglCreatePlatformWindowSurfaceEXT to return a nullptr for EGLSurface in xcb_xlib.cpp, exactly what we see in Qt.
Is there something we're missing in qt.cpp to make this work or is this a bug in Qt that can be fixed?
We started looking into the Qt source code, the call to eglCreatePlatformWindowSurfaceEXT(EGL_PLATFORM_X11_KHR, xlib display, [...]) returns 0. If we modify Qt to call eglCreatePlatformWindowSurfaceEXT(EGL_PLATFORM_XCB_KHR, xcb connection, [...]), we instead get a valid surface that can be queried with eglQuerySurface that report the correct size. The window is still black but having a surface is a good start for anyone looking into this.

Attached source files