The benchmark for QVector contains a "raw vector" test (originally introduced in Qt 4.7 to compare performance with that of std::vector) which does some positively devious hackery to overwrite the memory of a QVector with an ersatz object that has the same layout as a Qt 5 QVector.
Given the drastic change in the internal layout of QVector at Qt 6 (due to its conversion to QList, alongside major reworking of QArrayData), this ersatz object is no longer up to the job.

I shall disable the relevant test and tag it with this issue's ID, for re-enabling when this heinous hackery has been reworked.