The following examples will fail to launch when built with a static build of Qt. Both with qmake and CMake.

/Examples/Qt-6.0.0/quick/customitems/painteditem
/Examples/Qt-6.0.0/quick/imageprovider
/Examples/Qt-6.0.0/quick/imageresponseprovider
/Examples/Qt-6.0.0/qml/qmlextensionplugins
/Examples/Qt-6.0.0/qml/tutorials/extending-qml/chapter6-plugins
/Examples/Qt-6.0.0/qml/tutorials/extending-qml/chapter6-plugins/import

Copying short intro about investigation and direction of fixes

Static Qt + qml plugin examples don't work in qmake due to what Fabian mentions.
We need to add classname entries to the qmldir files. But even that is not enough. I suspect we'll have to implement support for pure qml modules in qmake, similar how i've done it for CMake in https://codereview.qt-project.org/c/qt/qtdeclarative/+/320052
I played around how far I can get with modifying the project to get it working with qmake, but got stuck due to the issue above.
Here's the diff and execution of the app https://paste.ubuntu.com/p/5VkC8FNjzb/