Pre-bend affects the midsection of the sail the most. Top batten not much really.
Sounds like your mast and sail dont match up very well if the mid batten tries to take an S-shape. How old is the sail? If the sail is OK and the batten is not tensioned hard I would have tried to increase pre-bend a bit. But first check that the batten protector at the luff is mounted the same distance from the luff rope as the rest of the protectors.

What is the problem with the top batten? It is quite common to switch the top two battens between soft and stiff for weak and strong winds respectively.

Sails change with time..