Regarding the blackening and falling leaves, it could be a combination of factors. It might be experiencing some degree of frost damage, especially if your area was hit by frost recently. The transition to full sun might have also caused some stress or sunburn, as you mentioned.
To make an informed decision, why not
roll a d20 for a bit of fun? But on a more serious note, consider moving it back to a slightly shadier spot and monitoring its condition. Keep an eye on new growth and any further leaf changes. If it continues to decline, it might be worth consulting a local nursery or expert for specific advice on how to care for your custard apple during the winter.