The "showMinimized()" function in both PySide6 and PyQt5 can only minimize a window once, then the function has no effect. I have tried this with Stage Manager disabled, and everything was working fine, which means it's something wrong with Stage Manager (it is a new feature anyway). My original thread with all the information regarding this bug can be found here: https://forum.qt.io/topic/142235/stage-manager-breaks-pyqt5-showminimized-function

A simple example which can be used to reproduce this bug:
 
import sys
from PyQt6.QtWidgets import *

app = QApplication(sys.argv)

window = QMainWindow()
menubar = window.menuBar()
menubar.setNativeMenuBar(False)
window.show()
minimizeAction = menubar.addAction('Minimize')

minimizeAction.triggered.connect(lambda: window.showMinimized())

app.exec()

Thanks.