I really am not sure. But trying to look and guess, maybe the initial detection of dark mode to call fixMacDarkModePalette in MainApplication isn't correct, so it is only called on palette change for switching to dark mode. Or maybe it's something else. Concerning detection of dark mode, there's this for 6.2:
https://codereview.qt-project.org/c/qt/qtbase/+/375648
Yet I use another Qt 5.15.2 app that has no issues of alternate row colors. It's doing something differently.
If that is it, one linked bug notes "Use new QPlatformTheme::appearance() function Instead of qt_mac_applicationIsInDarkMode()", of which the later I'm not sure what it's referring to. Another example is this using the native API:
https://successfulsoftware.net/2021/...t-application/