I guess if there had been concern of a leak, that was apparently a false positive of a unit test. (I'm not sure about this detail).
This has been resolved, as of today. Here is a summary:
This bug originally appeared in Qt 5.0, was reported in Qt 5.3.0 (QTBUG-39147), and fixed for Qt 5.9.2 (on 7-19-2017). The removal of Qt_WS_* macros (replaced with Q_OS_* macros), and incomplete porting of the widgets/kerna/qtooltip.cpp module, caused the QTipLabel to be instantiated with the client widget as its parent widget. That had been a known problem on Windows which directly caused this dysfunction (Showing a tooltip raises the whole window).
See "Patch Set 6" in this Qt code review page, and it's associated "gitweb" link, and the original bug report: