code example:

QJsonObject testObject;
qInfo() << testObject["keyname"].isUndefined() << testObject["keyname"].type();

This is code print "false 0".
where "false" is result of testObject["keyname"].isUndefined()
"0" is result of testObject["keyname"].type() and equal QJsonValue::Null

In documention printed:

QJsonValue QJsonObject::operator[](const QString &key) const
Returns a QJsonValue representing the value for the key key.
This does the same as value().
The returned QJsonValue is QJsonValue::Undefined if the key does not exist.

But returned value have type QJsonValue::Null, not QJsonValue::Undefined.

1. Вehavior does not comply with the documentation;
2. In some case QJsonValue::Null is correct value of object property.

Sorry for my english