In 6.3 transient workflow is introduced and update asset workflow is transient OOTB.
Standard workflows save runtime (history) information during their execution. You can also define a workflow model as Transient to avoid such history being persisted. This is used for performance tuning as it saves/avoids the time/resources used for persisting the information.
So the behavior is different in 6.3 than 6.0 and is expected.
DAM_UPDATE_ASSET_WORKFLOW_COMPLETED is internal event for Assets UI.
You can make workflow non-transient from workflow UI to get previous behavior.
https://helpx.adobe.com/experience-manager/6-3/sites/developing/using/workflows.html#TransientWorkfl...