On a Q_GADGET with a qreal property, like:

class TestType
{
    Q_GADGET
    Q_PROPERTY(qreal value READ value WRITE setValue RESET resetValue)

This assignment:

test.value = undefined

Calls setValue(NAN) in 6.5.0 instead of resetValue() as it did in 6.4.1 and earlier.

This still works as expected (calling resetValue()) for Q_OBJECT properties or for int properties of a Q_GADGET.