Run the following application:

import QtQuick 2.0

Rectangle {
    width: 640
    height: 480

    TextInput {
        anchors.centerIn: parent
        width: parent.width
    }
}

Choose an input method with pre-edit text and input some text. The text will not display until you commit it. Do the same, and the input will display correctly. If you initialize the input field with some text, the pre-edit text will display on the first try, regardless of whether it's prepended or appended to the existing contents, so this is only reproducible when the text input is completely empty.