Is there a link to an official comment that this is working as intended? If not then this is just another bug to add to the ever going list of frustrating issues with game mechanics. Stealth detection beyond the range of normal aggro range is wrong. If they wanted to deter people from stealthing past content perhaps they should have the NPC's have increased stealth detection within the area that they would normally aggro someone. My thought is that they tried to do just that and ended up with broken detection.