Issue with QMediaDevices::videoInputs(): return empty list but Qt Camera example works fine

Question

I'm trying to use QMediaDevices just to grab the list of cameras input. QMediaDevices::videoInputs() return an empty list; no error is thrown in compilation time. On run-time I got a sort of warning / log:

Available HW decoding frameworks:
     dxva2
     d3d11va

Strange thing being, if I load the Qt example Camera, everything (including the list of my devices) work fine.

That's what I've got so far:

#include "videomanager.h"

#include <opencv2/imgcodecs.hpp>  // I need opencv2 for other reasons
#include <QtNetwork>
#include <QThread>
#include <QMediaDevices>
#include <QCameraDevice>
#include <QCamera>
#include <iostream>

[...]

QJsonArray VideoManager::cameraList() const
{
    //QCamera *camera = new QCamera; // // As suggeste by https://stackoverflow.com/a/43326541/5321862 - Not working
    const QList<QCameraDevice> cameras = QMediaDevices::videoInputs();
    QJsonArray json_cameras;
    for (int i = 0; i < cameras.length(); i++) {
        QCameraDevice camera = cameras[i];
        QJsonObject o;
        o["description"] = QJsonValue(camera.description());
        o["index"] = QJsonValue(i);
        o["id"] = QJsonValue(QString(camera.id().toBase64()));
        json_cameras.append(QJsonValue(o));
    }
    return json_cameras;
}

My .pro file (automatically generated) follows:

QT -= gui
QT += network multimedia
requires(qtConfig(udpsocket))

CONFIG += c++17 console
CONFIG -= app_bundle

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
        definitions.cpp \
        main.cpp \
        message.cpp \
        tcpconnectionmanager.cpp \
        videograbber.cpp \
        videomanager.cpp

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

HEADERS += \
    definitions.h \
    message.h \
    tcpconnectionmanager.h \
    videograbber.h \
    videomanager.h

win32:CONFIG(release, debug|release): LIBS += -L$$PWD/../../local/custom-libs/lib/ -lopencv_core470
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/../../local/custom-libs/lib/ -lopencv_core470d
else:unix:!macx: LIBS += -L$$PWD/../../local/custom-libs/lib/ -lopencv_core470

INCLUDEPATH += $$PWD/../../local/custom-libs/include
DEPENDPATH += $$PWD/../../local/custom-libs/include

win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/libopencv_core470.a
else:win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/libopencv_core470d.a
else:win32:!win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/opencv_core470.lib
else:win32:!win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/opencv_core470d.lib
else:unix:!macx: PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/libopencv_core470.a

win32:CONFIG(release, debug|release): LIBS += -L$$PWD/../../local/custom-libs/lib/ -lopencv_videoio470
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/../../local/custom-libs/lib/ -lopencv_videoio470d
else:unix:!macx: LIBS += -L$$PWD/../../local/custom-libs/lib/ -lopencv_videoio470

INCLUDEPATH += $$PWD/../../local/custom-libs/include
DEPENDPATH += $$PWD/../../local/custom-libs/include

win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/libopencv_videoio470.a
else:win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/libopencv_videoio470d.a
else:win32:!win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/opencv_videoio470.lib
else:win32:!win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/opencv_videoio470d.lib
else:unix:!macx: PRE_TARGETDEPS += $$PWD/../../local/custom-libs/lib/libopencv_videoio470.a

DISTFILES += \
    defaultConfig.json

Edit

A call to QMediaDevices::audioInputs() before videoInputs() solves the problem, but I can't figure out why and, above all, I think a useless call is not an actual solution

#include <QAudioDevice>
[...]
const auto audios = QMediaDevices::audioInputs(); // Useless but allows videoInputs() to return the right camera.
const QList<QCameraDevice> cameras = QMediaDevices::videoInputs();

Furthermore, I've opened an issue on Qt

Edit 2

Issue has been solved