When an item with controls in the item template is collapsed, the AJAX settings are broken as the control's DOM element is moved because the MoreButton dropdown is not part of the Toolbar element itself. The solution is to set the overflow property of the item to Never - <telerik:RadToolBarButton OverFlow="Never"> https://docs.telerik.com/devtools/aspnet-ajax/controls/toolbar/mobile-support/responsive-design#preventing-items-from-being-collapsed
Replicated only when Orientation=Vertical. The problem seems to be the _responsiveCheck function of Toolbar\Views\LiteView.js called from the _repaint method of the same file. screencast: https://www.screencast.com/t/SF5b1KZR7pIp screenshot before/after https://www.screencast.com/t/KG645ZgX Workaround: calling toolbar.get_view()._showAllHiddenItems(); after adding programmatically the buttons.