Why does qml files fail to load if I separate the entry?

? Offline

I have the following directory:

.
├── CMakeLists.txt
└── src
    ├── CMakeLists.txt
    ├── main.cpp
    └── radon
        ├── main.qml
        ├── qml.qrc
        ├── radon.cpp
        └── radon.h

./CMakeLists.txt :

cmake_minimum_required(VERSION 3.15)

project(Radon LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

add_subdirectory(src)

src/CMakeLists.txt is:

set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)
find_package(Qt5 5.15 REQUIRED COMPONENTS Core Quick REQUIRED)
add_library(RadonCore
        radon/radon.h
        radon/radon.cpp
        radon/qml.qrc
        )

target_compile_definitions(RadonCore
        PRIVATE $<$<OR:$<CONFIG:Debug>,$<CONFIG:RelWithDebInfo>>:QT_QML_DEBUG>)
target_link_libraries(RadonCore
        PUBLIC Qt5::Core Qt5::Quick)

add_executable(RadonApp
        main.cpp)
target_link_libraries(RadonApp
        PRIVATE RadonCore)

Right now, the radon.h simply contains the entry to my application, its definition is as follows:
radon.h:

#ifndef RADON_RADON_H
#define RADON_RADON_H

namespace radon{
    int radon_entry(int argc, char* argv[]);
}

#endif //RADON_RADON_H

radon.cpp:

#include "radon.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>

int radon::radon_entry(int argc, char* argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
            &app, [url](QObject *obj, const QUrl &objUrl) {
                if (!obj && url == objUrl)
                    QCoreApplication::exit(-1);
            }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

The main.cpp just calls the entry point:

#include "radon/radon.h"

int main(int argc, char* argv[])
{
    return radon::radon_entry(argc, argv);
}

The idea is to provide my own main in tests. However, Qt complains about not able to find main.qml:

QQmlApplicationEngine failed to load component
qrc:/main.qml: No such file or directory

If I provide a main in radon.cpp and build it as an executable then it is able to find the main.qml. Why is this so? How can I separate the entry to my app?

qml.qrc is as follows:

<RCC>
    <qresource prefix="/">
        <file>main.qml</file>
    </qresource>
</RCC>