QPlainTextEdit set background not working in QGraphicsProxyWidget

[Niagarer]

Hi,
I'm sorry, I know these 'fix my code' questions are often quite useless, but I am really stuck, my apologies.
I am trying to set the background-color of a QPlainTextEdit via stylesheet and it doesn't work. Setting the color does work though. The widget gets placed with a QGraphicsProxyWidget in a QGraphicsScene. But from what I know, this shouldn't affect the visual appearance of the widget.

class Code_NodeInstance_MainWidget(QWidget):
    def __init__(self, parent_node_instance):
        super(Code_NodeInstance_MainWidget, self).__init__()

        self.code_text_edits = []

        self.code_font = QFont('Courier New', 12)

        # UI
        self.setLayout(QVBoxLayout())

        self.add_new_script()


    def add_new_script(self):
        code_text_edit = QPlainTextEdit()
        code_text_edit.setPlainText('test code')
        code_text_edit.setFont(self.code_font)
        code_text_edit.setStyleSheet('background: black; color: grey;')
        self.code_text_edits.append(code_text_edit)
        self.layout().addWidget(code_text_edit)

it just does not set the background, I also tried using

self.setStyleSheet('''
QPlainTextEdit {
    background-color: black;
    color: grey;
}
''')

in the constructor, also didn't work.
I am probably stupidly missing something pretty obvious.
Thanks for answers!

edit
I am very confused. The following two prints:

        print('before: ', self.height())
        self.layout().addWidget(code_text_edit)
        print('after: ', self.height())

print exactly the same also when I can see that the widget expands vertically and I think that shouldn't be.
You can test this, MRO:

from PySide2.QtWidgets import QMainWindow, QApplication, QWidget, QVBoxLayout, QPlainTextEdit, QGraphicsView, QGraphicsScene, QGraphicsProxyWidget

class CodeWidget(QWidget):
    def __init__(self):
        super(CodeWidget, self).__init__()

        # UI
        self.setLayout(QVBoxLayout())

        self.add_new_script()

    def add_new_script(self):
        code_text_edit = QPlainTextEdit('test code')
        code_text_edit.setStyleSheet('background: black; color: grey;')
        print('before: ', self.height())
        self.layout().addWidget(code_text_edit)
        print('after: ', self.height())

    def mousePressEvent(self, event):
        print('mouse pressed! adding new script')
        self.add_new_script()


class MyView(QGraphicsView):
    def __init__(self):
        super(MyView, self).__init__()


        # create UI
        scene = QGraphicsScene(self)
        scene.setSceneRect(0, 0, 500, 500)

        self.setScene(scene)

        self.code_proxy = QGraphicsProxyWidget()
        self.code_widget = CodeWidget()
        self.code_proxy.setWidget(self.code_widget)

        scene.addItem(self.code_proxy)
        self.code_proxy.setPos(50, 50)


class MainWindow(QMainWindow):
    def __init__(self):
        super(MainWindow, self).__init__()

        w = MyView()

        self.setCentralWidget(w)



if __name__ == '__main__':
    app = QApplication(sys.argv)

    mw = MainWindow()
    mw.show()

    sys.exit(app.exec_())

just click on the little space between text edit border and CodeWidget border. I don't know if these things are related but maybe my widget is fundamentally wrong configured.