QFileDialog always opens behind main window

I'm trying to open a file in my PySide2 application, but the file dialog always opens below the main window and appears as another application in the launcher. The application's name is "Portal".

I see other answers where the solution is to pass the main window as the first parameter to getOpenFileName(), but that doesn't work for me.

Here's a simple demonstration of the problem:

import sys
from PySide2.QtWidgets import QPushButton, QFileDialog, QApplication


class DemoButton(QPushButton):
    def __init__(self, text):
        super().__init__(text)
        self.clicked.connect(self.on_click)

    def on_click(self):
        file_name, _ = QFileDialog.getOpenFileName(
            self,
            "Open a text file.",
            filter='Text file (*.txt)')
        print(file_name)


def main():
    app = QApplication(sys.argv)
    button = DemoButton("Hello World")
    button.show()
    app.exec_()
    sys.exit()


main()

I thought maybe the parent had to be a QMainWindow, so I tried that:

import sys

from PySide2 import QtWidgets


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        main_widget = QtWidgets.QWidget(self)
        self.setCentralWidget(main_widget)

        # layout initialize
        g_layout = QtWidgets.QVBoxLayout()
        layout = QtWidgets.QFormLayout()
        main_widget.setLayout(g_layout)

        # Add Widgets
        self.exec_btn = QtWidgets.QPushButton('Execute')
        self.exec_btn.clicked.connect(self.find_file)

        # global layout setting
        g_layout.addLayout(layout)
        g_layout.addWidget(self.exec_btn)

    def find_file(self):
        file_name, _ = QtWidgets.QFileDialog.getOpenFileName(
            self,
            "Open a text file.",
            filter='Text file (*.txt)')
        print(file_name)


def main():
    app = QtWidgets.QApplication(sys.argv)
    window = MainWindow()
    window.show()
    app.exec_()
    sys.exit()


main()

The file dialog behaved exactly the same.

I'm using PySide2 5.12.2, Python 3.6.7, and running on Ubuntu 18.04.

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