Due to QStyleSheetStyle restores palette (in polish call) to saved one when it was connected to QWidget (or its parents), widget palette get locked in one state.
Same situation with QWidget font is handled with using _q_styleSheetWidgetFont property, which is updated in every setFont call.
But for palette I think we can add call in QWidget setPalette to QStyleSheetStyle which will update customPaletteWidgets variable.

Bug reproducing is simple, code in attachment