When you create a new story, there is a prompt to select the classic or optimized design experience. This is due to new functionality to enhance the designer's user interface, particularly for chart building. Since this is a newer feature there are some limitations that will quickly go away.
When the Optimized Design Experience is used, story widgets will not refresh until they are viewed. This improves the performance when opening stories. When the user goes to a different page in the story, the chart will appear initially as a grayed out (ghosted) image which provides a preview of what it will look like until it is refreshed.
As changes are made to objects in the story, the Undo/Redo option will be available either in the ribbon or in the ...More options.