When "External Content" is selected as the Content type for a Content Item record (Service Catalog > Catalog Definitions > Content Items) the Short description field is not visible on the form.
The UI Policy "Set fields for content" is hiding the Short description field.
- Navigate to the UI Policy "Set fields for content" (OOB sys_id is d334b1220a0a0b1200974715fa2beb1b)
- Either deactivate this UI policy or modify it (i.e. add another condition that says Content type is External Content) to make the Short description field visible when "External Content" is selected as a Content Type.