Temporary workaround: Handle the OnClientLoad event of the SearchBox. Event handler implementation:

var $ = $telerik.$;
function onSearchClientLoadHandler(sender, args) {
    $(sender.get_inputElement()).focus(function () {
        var comboBoxes = Telerik.Web.UI.RadComboBox.ComboBoxes;
        for (var i = 0; i < comboBoxes.length; i++) {
            var combo = comboBoxes[i];
            if (combo.get_dropDownVisible()) {
                combo.toggleDropDown()
            }
        }
    });
}