Transpanrent effect isn't working well with QScrollArea

[Moonsea]

I'm studying PySide6.
I want to add transpanrent effect to a QScrollArea. So I use QGraphicsOpacityEffect.
Here is the code:

from sys import argv

from PySide6.QtCore import QTimer
from PySide6.QtWidgets import QLabel, QScrollArea, QFrame, QGraphicsOpacityEffect, QApplication, QMainWindow


class Test(QMainWindow):
    timer = QTimer()

    def __init__(self, parent=None, *args, **kwargs):
        super().__init__(parent, *args, **kwargs)

        self.parent = parent

        label = QLabel('test\n'*20)
        label.setStyleSheet('font-size:20px;')

        scroll_area = QScrollArea()
        scroll_area.setWidget(label)
        self.setCentralWidget(scroll_area)

        opacity = QGraphicsOpacityEffect(scroll_area)
        opacity.setOpacity(0.5)
        scroll_area.setGraphicsEffect(opacity)

        self.show()


if __name__ == '__main__':
    app = QApplication(argv)
    ex = Test()
    exit(app.exec())

After running, there are some shadows and without transparent effect.
QGraphicsOpacityEffect works well with the widgets except QScrollArea.
Why?