QMenu separators are not visible with Qt5.15.5. This can be easily reproduced with Qt's Menu Example.
It seems that in QCocoaMenu::syncSeparatorCollapsable()
cocoaItem->setVisible(!previousIsSeparator &&
cocoaItem->isVisible());

makes separator visible only when it is not hidden from the collapsible
separator logic AND if it wasn't hidden before.

So setting setSeparatorsCollapsible(false) makes it work as workaround