The default is currently set to /org.qt-project/imports instead of /qt-project.org/imports. This creates a mismatch with the correct value assigned to the same thing in qt_internal_add_qml_module(). The correct value can be seen in qtdeclarative/src/qml/qml/qqmlimport.cpp.
One could argue that there should be no default for RESOURCE_PREFIX in the qt6_add_qml_module() command. In user projects, does it really make sense to use a resource prefix containing qt-project or a variation of it?