While flying forward, it takes more than a second from the moment you let go of the "w" to the moment until you stand still. When you need to precision-place blocks (which is often the point of flying around in creative), you can't really do that effectively until you stand still in the air.
The way you stop after running on the ground is much more reasonable.
This is IMO working as intended. The movement in the air is quite similar to the movement on ice.
Probably it's a personal preference what's suiting better, I like the soft movement in the air.
Nevertheless, this is no bug.