It will not work if you hold W+D (forward+right) or W+A (forward+left) entire time, and try aiming down sights multiple times. It only works if you go forward (W) and on the first switch of direction (adding left or right while still moving forward) press aim down sights.
It's easy to reproduce, but very weird to explain.
May 24th - EDIT: It would appear that this change in direction also causes the game to ignore your attempt to change stance, while holding a launcher.
How to reproduce:
-Take RPG42, or any other launcher
-As you move forward (W), press left (W+A), and immediately try to switch stance to crouch or standing. Game will always ignore you. It works with other directions as well, for example going from forward right (W+D) to just forward (W).
If you are holding a rifle and try that, the game will not ignore you, it will change the stance, but it will be delayed - opposed to changing stance without switching direction.