Different output on different macOS machines

[Strangelove]

On my development machine (iMac running Catalina set to Dark Mode), my app looks like this:

&

(Yes, the icons are black, that's another problem...)

If I run the exact same code on my laptop (Intel MacBook Pro running Big Sur set to Dark Mode as well), I don't get anything resembling Dark Mode, but I do get an unreadable dark font:

&

The toolbar is the default QMainWindow toolbar, no styling applied.

The blue bit is a completely separate QWidget with nothing but the following stylesheet applied:

MyWidget {
            background-color: RoyalBlue;
            border: 1px solid MediumBlue;
            border-radius: 8;
        }

MyWidget QLineEdit, QTextEdit, QSpinBox {
            background-color: MediumBlue;
        }

Any idea on what could be the cause of the different behaviors between machines? And maybe on how to avoid this?

Thanks.

EDIT: the dark color scheme above also happens when Big Sur is set to Light Mode.

[Strangelove]

@Strangelove Apparently this was a bug in Big Sur, fixed when upgrading from 11.4 to 11.5.2.