When configuring a palette for a button (standard one, no styling beside the palette), the buttonText controls text color, but all other palette fields mentioned in button.qml are not functional.

Here is a nonsensical example to display the problem:

    Button {
        id: control // named like this just for later test below.
        text: qsTr("I am ready!")
        palette {
            button: "red"
            buttonText: "red"
            dark: "red"
            mid: "red"
            highlight: "red"
        }
    }

Here is the visual result on Windows (file is attached as test.qml, can be opened with qml.exe):

Looking at Button.qml, I don't see what is causing the bug. palette.buttonText works, palette.button does not. They are structurally at the same depth, seemingly referencing the same control thing.
Strangely, if we copy-paste the background part of Button.QML (hopefully, I got the right file; there as so many with this name. I chose the one in "Basic" folder) in the button itself below palette, it works as we would expect. (file is attached as test2.qml can be opened with qml.exe)