If you are bringing your drone down with full down stick you are gonna get some fluttering for sure. This is in the design of the motors to break up all the prop wash so bird won't get caught in it's own wash and fall out of the sky. If you look closely each motor has a slight tilt or cant to them. But if you come down a little slower you won't get it.
It has nothing to do with that, you can still get into a Vortex if you come down fast, but that is limited by a maximum descent speed in the craft, the only way to get out of it, is to increase down throttle, and sideways. Throttle up more trying to fight it, will only make it worse.
Angled motors is known as Dihedral.
Dihedral effect of an aircraft is a rolling moment resulting from the vehicle having a non-zero angle of sideslip, youll find that on many fixed wing airplanes, seeing it from the rear or front, the wings have a V-shape.
On a multicopter, in short, this means, that when your craft is moving forward, the front motors will have a more horizontal angle, and the rear more tilted, that way the front motors will generate more lift, while the rear motors produce forward thrust, its an efficiency thing, or pure aerodynamic mathematics if you like.
Although fixing things on your own can be great, i would check with Dji first, if such operations will void the warranty or not, cause if something happens later, and they find parts have been tampered with, they could hold it against you, and say, this is not performed by a certified technician.
Make sure you get those approvals trough a written mail or letter
