Setting background-color for QToolTip does not work unless border is specified

1. Run the following code:

   #include <QApplication>
   #include <QToolButton>
   
   int main(int argc, char *argv[])
   {
       QApplication a(argc, argv);
       QToolButton button;
       button.setStyleSheet( "QToolTip {border: 0px;\
                                        background-color: rgb(0, 255, 0);}");
       button.setToolTip("my tooltip");
       button.show();
   
       return a.exec();
   }
   

2. Hover the button.
   You'll see a green tool tip:
   
3. Remove "border: 0px;" from the style sheet.
4. Run the application and hover the button:
   The tool tip shows up in default color:
   

My understanding is that the background color should still be green.