In ArcGIS Instant Apps, the Sidebar template includes the Details setting to display the description content from the source map's item details in the side panel of the app. However, in some instances, the edits made when editing the HTML Source section in the Sidebar template fail to be saved.
The image below shows the OK button is not available in the Sidebar template in ArcGIS Instant Apps to save the edits.
This is by design. This issue occurs when the editing mode for the HTML Source section is enabled.