Insight Objects can have a User type attribute shows the User in the UI.
The "User" Attribute value is saved in the Insight DB as a Text value, containing the userKey or, the Username.
When compiling to show this value in the UI, Insight will perform a User lookup.
If the user was deleted from Jira - the lookup will not find a user, but as there is a Value saved for the Attribute - it will show as an "Unknown" user.
Whenever such an object is attempted Update - the update will fail as the User is invalid (also affecting Bulk Update).

Feature request: 

As a User, in case the User lookup fails (since the User was deleted) - it will be beneficial to ignore the User Attribute, and allow the object Update to be saved - so the workflow is not broken.