Hello,
here is a theory for you: the load from the mainsheet is traveling in a direct line from the boom bail to the sail head. The roach of the sail is falling to leeward.
something is mismatched. mast stiffness or bend vs. sailcut is an obvious place to look. battens that are tubular in the back may help. fine tuning the various tensions involved may be all that is necessary.
have fun with it.