It may be that you have no restrictions which is good.
Is there any nerve compression or nerve damage?
You may need to retrain the muscles if you had an injury.
It may be establishing and maintaining a stable position that needs attention.
In a static position you may be able to set the position, but what happens when movement is introduced?
Category 2/3 movements vs category 1 movement.
It could be understanding what is needed to create the stable position.
It could be a motor learning situation.
The front of your shoulder may be tight so when you flex your back your body has to find slack within the system to allow you to flex your back. What you are seeing is how your body is compensating to complete the task.
This compensation can create other issues up/down stream.