Relying on the when property of the State QML Type to handle state changes, leads to the default "" state being applied.

Consider the following example: 

Item {
    id: item
    anchors.fill: parent

    property bool toggle

    Button {
        anchors.fill: parent
        onClicked: item.toggle = ! item.toggle
    }

    states: [
        State {
            name: "true"
            when: item.toggle
        },
        State {
            name: "false"
            when: ! item.toggle
        }
    ]

    onStateChanged: {
        console.log("state:", state, "toggle:", toggle)
    }
}

The expected output when clicking the Button twice is:

qml: state: false toggle: false
qml: state: true toggle: true
qml: state: false toggle: false

where the first line appears at startup. This output is obtained with QT 5.12.9.

With QT 5.15.2 one gets the following output: 

qml: state: false toggle: false
qml: state:  toggle: true
qml: state: true toggle: true
qml: state: false toggle: false

The second line shows that the transition from the "false" state to the "true" state temporarily applies the default "" state in between. This in particular leads to transitions from: "false" being stopped.

When there are only 2 states a workaround is to specify "when: true" in the second state in the list. For QT 5.12.4 until QT 5.14.0 even that does not work, because of QTBUG-76838, but "when: 1 == 1" works in these versions. For more than 2 states one needs to handle state changes in javascript.

Windows: reproduced, iOS: reproduced