That report (
https://bugreports.qt.io/browse/QTBUG-84462) shows a patch to fix it. I checked that patch against Qt-6.5.2 sources and that fix is already in Qt6.5.2.
So it must be some other bug.
That bug was specific to QToolButtons and not QMenu's. Your image shows a QMenuBar QMenu showing up on the wrong screen.
My guess is that your "Primary Screen" is set in Windows Display to be the screen on the right.
Are you by chance launching Sigil on that screen on the right first, and then manually moving it to the left screen?
If so try moving it back to the right screen before closing Sigil. and repeat the test.
Does it show any difference when you change the screen's relative locations in Windows Display control panels?
What dpi are your monitors and what scalings are you using?