If map units are set to metric in the ArcGIS Online Organizational settings, and a user accesses a Web AppBuilder application outside of the organization, the units of the scale bar are imperial and do not reflect the map unit setting.
When creating the web application:1) Under Widgets > open the Scalebar widget.2) Do not make any changes (unless desired), click OK.3) Save the application.This process updates the JSON for the widget to include the scalebar unit.