In the meantime, use the following workaround:

<script type="text/javascript">
                var designMode = Telerik.Web.UI.EditModes.Design;
                var editorPrototype = Telerik.Web.UI.RadEditor.prototype;
                var keypress = editorPrototype._keyPressHandler;
                editorPrototype._keyPressHandler = function (ev) {
                    if (this.get_mode() == designMode) {
                        keypress.call(this, ev);
                    }
                }
                var keyDownOverride = editorPrototype._keyDownHandlerEnableTrackChangesOverride;
                editorPrototype._keyDownHandlerEnableTrackChangesOverride = function (ev) {
                    if (this.get_mode() == designMode) {
                        keyDownOverride.call(this, ev);
                    }
                }
                var keydown = editorPrototype._keyDownHandler;
                editorPrototype._keyDownHandler = function (ev) {
                    if (this.get_mode() == designMode) {
                        keydown.call(this, ev);
                    }
                }
            </script>