javascript drag and drop not offsetting dragimage when using touch input

I am using html5 drag and drop functionality in a project. I am trying to alter the position of the drag image relative to the position of the input (mouse or finger). The following code works fine on a desktop when using a mouse, however when using a touch panel on a phone screen, the image is never offset.

I originally thought it might be a screen pixel-density issue, but even when I replace the offset with much bigger numbers, nothing happens.

    function dragStart(ev)
    {
        console.log("dragging");
        console.log(ev);

        // Place the drag image offset to the mouse cursor location
        ev.dataTransfer.setDragImage(ev.target, 50, 50);

        ev.dataTransfer.setData("text", ev.target.dataset.tileIndex);
    }