The video below shows one cause of it. Or read below.
(When you change stances going from standing to crouch or crouch to prone even when fully ADSing, your accuracy will go horrible and actually WORSE than hipfiring sometimes.)
Another cause is ADS moving delay on this weapon. When you ADS while moving, there's a 0.2 second delay until your bullets will go right where you aim. If you shoot before that 0.2 second delay even when fully ADSed, it'll be like as if you were hip-firing.
Yes, this weapon is broken in a way, and could use some fixing/tweaking. These two problems I stated above need to be fixed/tweaked.