Root Cause

At the time the User Visibility record was created, both the user and the domain were present and valid. At some point, either the user or the domain were deleted, and that deletion did not cascade to this table. 

Alternately, a domain was imported to an instance and the accompanying reference records were not correctly imported.

EffectPerformance of queries is negatively impacted when references in the Domain column of sys_user_visibility are pointing to nonexistent or deleted domains, or the references to the user are pointing to nonexistent or deleted users.
Corrective Action
  1. Log onto your instance.
  2. Navigate to Domain Separation Center.
  3. Select the Errors widget.
  4. Open the User Visibility Invalid Records audit record.
  5. For each sys_id in the Details list on the right, either remove the record or repair the record to point to valid references.