If you are hitting the target aft of the bullseye on quartering away shots you are likely hitting it perfectly, you should be more worried about where the arrow is passing through the core of the target (and real animal) than necessarily hitting the bullseye on the target.
You may be mentaly compensating for that if you consistently hit aft of the bullseye on quartering away shots on the target.
On an animal that has past you (quartering away) and stopped, the animals opposite front leg is a good reference point for aiming because using that oposite front leg as a target reference will ensure you at least hit the lungs.
As this picture below shows on an animal quartering away the bullseye is in line with the front leg on the oposite side of the animal.
So if you are using a 3D animal target for your practice target it probably doesn't have a oposite leg to use to line up and so you can only imagine where the leg should be on the practice target, you also may not want to practice that shot much by shooting outside aft of the replacable core or you may damage the target but just keep it in mind on shooting the real thing in the field.
If you meant something else, please elaborate.