Steps to reproduce:

1. start an app with light style active
2. open the QFileDialog (doesn't matter if it started from cpp code or via QML's FileDialog)
3. close the file dialog
4. switch to another app
5. switch back to our app ---> palette brakes here, QStyleHints::colorScheme now reports "Dark"

As a result part of colors are switched to "dark" (e.g. menus, drawers, dialogs, switch backgrounds, etc) while others stay at "light".

I used a "gallery" example to reproduce this. Insert menu item that will open FileDialog, and print colorScheme into Label.
 

import Qt.labs.platform as Platform
...
    Platform.FileDialog {
        id: fileDialog
    }
...
                    Action {
                        text: "File open"
                        onTriggered: fileDialog.open()
                    }
...
            Label {
                text: "SCHEME: " + Qt.styleHints.colorScheme

Switching apps back and forth w/o previously open the file dialog doesn't make an issue.

Opening and closing file dialog w/o restoring the app doesn't make an issue too.