If e.g. the Material style is not explicitly imported (in a static build) in QML, the plugin won't be picked up by qmake/qt.prf's static QML import support and the default style will be used instead.
Either users explicitly import the styles they wish to use with the application, or we add dependencies to each style to controls' qmldir so that they don't need to. The latter is not really an option, because it means they'd get plugins they didn't ask for. So, we should document this for users who are doing static builds.