gcc linker: undefined reference to a symbol while target lib only contains a superclass method

I'm having repeatedly a linker error in Qt-Project containing a local Installation of Qt 5.14.2 and OpenCV 4.3 and a gcc 11 (fedora 35, x64) and in the last step bringing everything together there might be something off or not fit together:

This is the last call out of the make system together with the error:

g++ -Wl,-rpath,/home/qt/Qt/5.14.2/gcc_64/lib -o QtOpenCVCompatibleTest main.o mainwindow.o moc_mainwindow.o   -L/home/qt/Qt/5.14.2/gcc_64/lib -lopencv_core -lopencv_videoio -lopencv_highgui /home/qt/Qt/5.14.2/gcc_64/lib/libQt5Widgets.so /home/qt/Qt/5.14.2/gcc_64/lib/libQt5Gui.so /home/qt/Qt/5.14.2/gcc_64/lib/libQt5Core.so -lGL -lpthread
/usr/bin/ld: /usr/lib/gcc/x86_64-redhat-linux/11/../../../../lib64/libopencv_highgui.so: undefined reference to `QPushButton::hitButton(QPoint const&) const@Qt_5'

So in short: CV needs a symbol that Qt can't deliver.

The relevant parts of the .pro file:

QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

INCLUDEPATH += /usr/include/opencv4/

LIBS += -L/home/qt/Qt/5.14.2/gcc_64/lib -lopencv_core -lopencv_videoio -lopencv_highgui

A closer look towards the libs:

 nm -gDC /usr/lib64/libopencv_highgui.so | grep QPushButton
                 U QPushButton::paintEvent(QPaintEvent*)@Qt_5
                 U QPushButton::qt_metacall(QMetaObject::Call, int, void**)@Qt_5
                 U QPushButton::qt_metacast(char const*)@Qt_5
                 U QPushButton::focusInEvent(QFocusEvent*)@Qt_5
                 U QPushButton::focusOutEvent(QFocusEvent*)@Qt_5
                 U QPushButton::keyPressEvent(QKeyEvent*)@Qt_5
                 U QPushButton::staticMetaObject@Qt_5
                 U QPushButton::event(QEvent*)@Qt_5
                 U QPushButton::setFlat(bool)@Qt_5
                 U QPushButton::QPushButton(QWidget*)@Qt_5
                 U QPushButton::QPushButton(QWidget*)@Qt_5
                 U QPushButton::~QPushButton()@Qt_5
                 U QPushButton::minimumSizeHint() const@Qt_5
                 U QPushButton::sizeHint() const@Qt_5
                 U QPushButton::hitButton(QPoint const&) const@Qt_5
                 U typeinfo for QPushButton@Qt_5

So yes, we need QPushButton::hitButton(QPoint const&) of Qt in opencv's highgui

but the Qt Version only has:

nm -gDC /home/qt/Qt/5.14.2/gcc_64/lib/libQt5Widgets.so | grep hitButton
0000000000331a00 T QToolButton::hitButton(QPoint const&) const@@Qt_5
00000000002e4990 T QRadioButton::hitButton(QPoint const&) const@@Qt_5
000000000024e060 T QAbstractButton::hitButton(QPoint const&) const@@Qt_5
000000000025d840 T QCheckBox::hitButton(QPoint const&) const@@Qt_5

... the method of its superclass, which in fact should be enough to have something to bind to.

So there are 3 Questions left:

1. How does the early binding in c++ works along the linker - I would guess every inherited class have at least a symbol pointing towards a proper method. I've might build OpenCV with a different version of Qt (but all Qt5 Versions have QPushButton::hitButton)
2. What is the correct calling order in gcc's linker command - I assume all Qt libs will be always called last as long as every other component depends on them?
3. Basically: How do I get rid of that error.

The local Qt Installation (on a Fedora 35 x64) is located as its own user but with all files at least accessible. A pure Qt Project just builds fine on the system.