Designer files are missing from cmake build. See API review https://codereview.qt-project.org/c/qt/qtquick3d/+/317708
This also affects qtquickcontrols2.
AUX_QML_FILES is an undocumented qmake variable that's used by qtbase/mkspecs/features/qml_module.prf to install the files, without embedding them into resources.
The CMake port is currently lacking a mechanism to handle that, and qt6_target_qml_files in its current state should not be used for that.
pro2cmake should also be adjusted to handle the variable assignments if possible.