Thanks for reporting. We'll fix it. Before we release a fixed version, you can first add elements to sortable-list and then init Sortable:

// Sortable
const sortableEl = document.getElementById('sortable');
for(i=1;i<=5;i++){
    const newElement = document.createElement('div');
    newElement.classList.add('sortable-item');
    newElement.textContent = 'New Element '+i; 
    sortableEl.append(newElement);
}
const instanceSortable = new DragAndDrop.Sortable(sortableEl);