Took me months of troubleshooting to solve this. Below is an example of FontLoader:

FontLoader {
    id: myFont
    source: "myFont.ttf"
}

When you load the .qml file containing above code using qml.exe, it works fine. When you put the above code in .qml file within a Qt Quick project, and run using Qt Creator, the font doesn't load (despite copying myFont.ttf to bin directory). If you use a resource file, it still won't load. Keeps throwing the error, "QML FontLoader: Cannot load font...", or something similar. The issue is with the way resource and fonts files are processed and then embedded into the binary by CMake.

[Solution 1]

1. Creat a resource file and add the font to it. i.e. resources.qrc:
   <RCC>
       <qresource prefix="/fonts">
           <file>myFont.ttf</file>
       </qresource>
   </RCC>
2. Add the resources.qrc to qt_add_qml_module() under RESOURCES:
   qt_add_qml_module(applearn-qt-quick
       URI learn-qt-quick
       VERSION 1.0
       QML_FILES
           Main.qml
       RESOURCES
           resources.qrc
   )
3. Set CMAKE_AUTORCC variable to true in CMakeLists.txt:
       set(CMAKE_AUTORCC ON)
4. Reference the font using the qrc url:
       source: "qrc:/fonts/myFont.ttf"

[Solution 2]

1. Creat a resource file and add the font to it. i.e. resources.qrc:
   <RCC>
       <qresource prefix="/fonts">
           <file>myFont.ttf</file>
       </qresource>
   </RCC>
2. Add the resources.qrc to CMakeLists.txt using qt_add_resources():
       qt_add_resources(RCC_SOURCES resources.qrc)
   Note: above line of code should be before qt_add_executable().
3. Add the RCC_SOURCES variable defined in step 2 to your qt_add_executable():
   qt_add_executable(applearn-qt-quick
       main.cpp
       ${RCC_SOURCES}
   )
4. Reference the font using the qrc url:
       source: "qrc:/fonts/myFont.ttf"
   Warning: Don't add resources.qrc to qt_add_qml_module().

[Solution 3]

1. Add the myFont.ttf to qt_add_qml_module() under RESOURCES:
   qt_add_qml_module(applearn-qt-quick
       URI learn-qt-quick
       VERSION 1.0
       QML_FILES
           Main.qml
       RESOURCES
           myFont.ttf
   )
2. Reference font using the follow url:
       source: "myFont.ttf"
               or
       source: "qrc:/qt/qml/learn-qt-quick/myFont.ttf"
       Note: replace learn-qt-quick with your own project name.

[Solution 4]

1. Put myFont.ttf in the same directory as the app executable binary.
2. Use the following url:
       source: "file:myFont.ttf"

Hope this help. Thank you for your time and have a great day!