An existing polygon's attributes are reset to the domain's default values. This occurs when the Auto Complete Polygon task is being used to create a new polygon feature in a Geodatabase featureclass with attribute domains and the domain's split policy is set to Default.
This is a known issue.
The work-around is to change the split policy for the attribute domain from Default to Duplicate. When the domain's split policy is set to Duplicate, any new polygons created using the Auto Complete Polygon will get the default values, and the existing polygons will retain their current attribute values.