How loose does that backstay get? Downwind, I ease my adjuster completely off. Some boats even use a fraculator to bend the mast even farther forward (granted, these masts are probably bendier than yours).
The point is that I think that the eased backstay must have exposed a pre-existing condition - probably a core issue or poor tabbing on the bulkheads. Unfortunately, this does sound like a major issue.
Good luck with it.
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