The decrease in fatigue is not a property that's automatically conferred to the plane by the introduction of wet wing - it's a product of smart design. Usually the decision to use a wet wing is made during the initial design of a plane, and the support structure of the wing is built accordingly, so that it can take the dynamic strains that occur upon refueling (in case of military aircraft - possibly in-air refueling) and upon spending fuel from the tanks. The initial design of the (initially dry-wing) B-52, obviously, did not account for putting such loads on wing structure (the additional internal wing tanks carried 17 tons of fuel!).
During the modification to wet wing, the wing should've been reinforced to counteract the additional load. Instead, the range of the bomber was prioritized: G and H models were lighter than previous models, mostly due to extensive usage of aluminium and titanium parts, particularly - in wing structure; at the same time they carried much more fuel. Alloys used were less fatigue-resistant than ones used in previous models. The testing done in 1960 by Boeing and Air Force confirmed that combined with additional stresses induced by change of usage (i.e. low altitude missions and wing tanks operation) this significantly shortened the operational life of the aircraft. This result was made painfully clear by B-52G that crashed near Gainsborough in January 1961. That led to a new modification program being endorsed by the Air Staff in May 1961, which was finished in 1964 - it involved reinforcing the wings and other areas subject to increased fatigue ("The wing structural improvement program, carried out as ECP 1050, replaced the wing box beam with a modified wing box that used thicker aluminum. It also installed stronger steel taper lock fasteners in lieu of the existing titanium fasteners; it added brackets and clamps to the wing skins, added wing panel stiffeners, and made at least a dozen other changes. Finally, a new protective coating was applied to the interior structure of the wing integral fuel tanks." - Encyclopedia of US Air Force Aircraft and Missile Systems: Post-World War II bombers, 1945-1973, Volume 2).