Have your player character simply stand (as opposed to crouching), and adjust your stance with Ctrl + w key to change stance to stand upright.
When your player character undergoes a jump animation, the stance changes to "default standing stance" right before the jump and the player character retain this "default standing stance" after the jump.
Edit: I realize now that this behaviour does not always occur. It might perhaps seem as if the inconstistent stance is linked to having used the "y" with arma 2 preset (combat pace toggle).