OpcUa Connection QML Type has "invalid" property "connection"

"connection" is a well-documented property:
https://doc.qt.io/qt-5/qml-qtopcua-connection.html#connection-prop
and also a registered one (in opcuaconnection.h):

Q_PROPERTY(QOpcUaClient* connection READ connection WRITE setConnection NOTIFY connectionChanged)

But somehow it is not valid:

The problem only exists in 5.15. Qt6 looks OK. Not sure what was patched, but that patch needs backport.