In the December 2016 release of ArcGIS Online, a functionality is added that allows administrators to change the organization short name. The short name uniquely defines the URL of an organization. For more information, refer to the Esri blog under the 'Organization administration' section: What's New in ArcGIS Online (December 2016). This effectively changes the organization URL and users may need to manually update URLs of hosted content and resources, including web apps.
The following is an example of a standard web application URL in comparison to the new URL.
New URL after organization short name is changed
However, web applications with new URLs continue to work even if the old URL is referenced. This is by design, as an application is referenced using only the Item ID. The old URL serves as a fallback, ensuring the server routes the user to the correct application, referencing the respective Item ID of an application. When an ArcGIS Online organization short name is changed, the user is recommended to update the URL of the hosted applications for branding.