I think what you are describing is a fix to allow the phone to display the "true" brightness when you are in control center. Before it didn't show you a true brightness so you had a hard time adjusting it. When you take your finger off the slider, it does dim down a bit.
But when you exit control center, I think the idea is that it will be what you saw/set in control center.
Yeah, from Anandtech review: Setting brightness from Control Center has also been improved. Bringing up Control Center dims the brightness of the app beneath it. On iOS 7 this meant that it was difficult to gauge how bright your display would actually be during use was when changing brightness from Control Center as the application below was darker than it normally would be. On iOS 8 the dimming effect is removed when changing brightness using the slider in Control Center, eliminating this issue.