Rotating bones in LateUpdate causes Character to flicker

I am currently trying to modify specific bones in my character to make him with his weapon to a specific point. Therefore, i want to bend his spine:

 void LateUpdate()
 {
     var targetTransform = transform;
     foreach (var bone in _pathToNeck)
         targetTransform = targetTransform.FindChild(bone);

     targetTransform.localEulerAngles += new Vector3(90,0,0);
 }

Unfortunatelly, the model starts to flicker regarding the animation. Half of the frames he is bended by 180 degrees (his spine points downwards), it seems that this code is executed twice (sometimes). Therefore, i checked the very same script with another character - and it works like a charm! Did i do something wrong during importing?

Update: If i do not change anything in LateUpdate, the animations run smoothly without flickering / strange behaviours.