I've had this problem on a number of builds and I have fixed the problem in the same way. I use an architect's scale, rubber bands and clamps. The architects scale is super stiff and it will not bend, so it makes the wing straight.
This is the wing on a 1/48 B-29. Both main wings were warped. The architects scale, rubber bands and clamps made the wing perfectly straight.
Same problem with a 1/48 B-58. Same solution.
1/48 B-24J had the same warped wing problem. The architects scale, rubber bands and clamped fixed the problem.