Calling the following Javascript from the OnSave event does work around the problem, but I still think the underlying Kendo needs to be fixed:

function Grid_FixDropDowns(e) {
        var dropdowns = e.container.find('[data-role=dropdownlist]');
        dropdowns.each(function (i, dd) {
            var fldval = e.model.get(dd.id);
            if (!fldval) {
                var val = $(dd).data().kendoDropDownList.value();
                e.model.set(dd.id, val);
            }
        });
}