For the time being you can add the following JavaScript workaround at the end of the form:

<script>
            Telerik.Web.UI.RadGrid.prototype.repaint = function () {
                if (!this.canRepaint()) {
                    if (typeof (this.get_element()) != "undefined") {
                        this.add_parentShown(this.get_element());
                    }
                    return;
                }
                else {
                    this._clearParentShowHandlers();
                }

                if (!this._scrolling && this.GridDataDiv) {
                    this._scrolling = $find(this.get_id())._scrolling;
                }

                if (Telerik.Web.UI.GridScrolling && this._scrolling) {
                    if (this._scrolling._shouldInitializeLayoutAndScroll) {
                        this._scrolling._shouldInitializeLayoutAndScroll = false;
                        this._scrolling._initializeDimensions();
                        this._scrolling._initializeScroll();
                    }
                    else {
                        this._scrolling.onWindowResize();
                    }
                }
                else {
                    if (this._repaintWidthFlag)
                        this.get_element().style.width = "";

                    var masterTableViewWidth = 0;
                    if (!!this._masterClientID && this.get_masterTableView() != null) {
                        masterTableViewWidth = this.get_masterTableView().get_element().offsetWidth;
                    }
                    if (this.get_element().offsetWidth < masterTableViewWidth) {
                        this.get_element().style.width = masterTableViewWidth + "px";
                        this._repaintWidthFlag = true;
                    }
                }
                if (this._masterClientID && this.get_masterTableView() && this.get_masterTableView()._virtualization) {
                    this.get_masterTableView()._virtualization.repaint();
                }
            };
        </script>