I made a simple test page based on the snippet code on your first post of this thread. Minor modification was made on your code and I was unable to repro the issue on my local test.

function button1_onclick() {
    //get the grid object
    var WebGrid1 = ISGetObject("WebGrid1");

    //get current active edit cell index
    var curCellIndex = WebGrid1.GetActiveEditCell().cellIndex;

    //get current active edit cell element
    var curCellElm = WebGrid1.GetActiveEditCell().ToCellObject().GetElement();
    var curCellElm2 = WebGrid1.GetSelectedObject().ToRowObject().GetCell(curCellIndex - 1).GetElement(WG40.BODY, WG40.HTMLCELL);

    //get next editable cell object
    var nextCellObj = wgGetNextEditableCell(curCellElm);

    //get next editable cell element
    var nextCellElm = nextCellObj.element;

    alert(nextCellObj);
    alert(nextCellElm.innerText);

    return true;
}

Please have the snippet code shown above tested on your end and let me know whether it helps or not.