I noticed since i want the model to be backed by a MVC API - application in the back.
However changing to a call on stop: 'itemsChanged' instead helped me for now ( i have to test further)
My version :
angular.module('ui').value('uiSortableConfig', {
sortable: {
connectWith: '.connectedSortable',
//update: 'itemsChanged',
stop: 'itemsChanged',
}
});