qnamespace.h istreated specially in the qtbase/src/corelib/CMakeLists.txt (from commit 138c0d04373b48164a31a031885381874a1f82c0, and more recently in 20bb59ca61e6e40e04092771d8f1116dc011f9bc) I suspect for bootstrapping purposes.
Moc is called manually, and the meta types exported manually to qt6core_metatypes.json but then that file is overwritten by the "normal" automatic output.
Thus that meta info is lost.

We should either create a separate json file, or somehow add that to files to collect from the normal meta info.