This is an old issue left over from ArmA 2.
If you have a binocular equipped and you change stance, the player character first puts away the binocular, pulls out his rifle, changes his stance, puts away the rifle and finally pulls out the binocular again.
This is problematic because the animation lasts way too long and locks the character in place, which means the player can't react quickly to incoming fire by quickly ducking behind cover and might die.
It would be nice if the player could change stance with bino in hands, without all the redundant animations.