Qt 6 / QtCharts: Spurious Quick test failures in namespaced builds (g++ 9.2)

QML tests in namespaced builds started to fail with g++ 9.2 when updating
to qtdeclarative/fb3552e0d152e963d74cccc7b36f4e618638b722:

FAIL!  : example::tst_qml-qtquicktest BarSeries::test_axes() Uncaught exception: Unknown method return type: QAbstractAxis*
   Loc: [/home/qt/work/qt/qtcharts/tests/auto/qml-qtquicktest/tst_barseries.qml(55)]

Note that QtCharts has a special namespace, but it is similar to qtdatavis3d, which works. Also I have not been able to reproduce this with g++ 8.3.

See changes updating the deps at https://codereview.qt-project.org/c/qt/qtcharts/+/293415