QML DropShadow + cache = broken rendering

This has always been this way and I have to assume something is wrong with either Qt docs or graphical effects.

DropShadow docs encourage use of the `cached` property: "This property allows the effect output pixels to be cached in order to improve the rendering performance. [...]"

But enabling it breaks rendering, and always has, for as long as I have known. Try this example:

import QtQuick 2.15
import QtGraphicalEffects 1.15

Item {

    width: 500
    height: 500

    DropShadow {
        source: blueRect
        anchors.fill: blueRect
        radius: 80
        samples: 50
        cached: true // or false
    }

    Rectangle {
        id: blueRect
        color: "blue"
        width: 100
        height: 100
        anchors.centerIn: parent
        visible: false
    }
}

I can't find any way to set up this DropShadow, with layer effects or not (which btw, OT, is also broken, as layer effect dropshadow seems to render the dropshadow on top or something).