>>297267716
Try the other method I mentioned. It will obviously mean your shoulders don't move in game anymore, but then the position of your shoulder bones won't have an influence on your avatar. 
I believe the issue is caused by the IK resolver thinking your shoulders are lower than they should be, usually because of height issues. You can verify if this is the case by rotating your shoulders downwards in Blender's pose mode and seeing if it matches up to the sagging you saw in game.