If your spring hanger bolts are free to rotate in the hanger mount, then you have an issue, provided the bolts were splined in the first place. If you have wet bolts, the bolts are splined to keep them from rotating in the spring hanger, which is a bad thing. If yours are free to rotate, no longer splined, they have spun at some point, for some reason. Not good. You might double check the spring hangers and make sure they are not worn, allowing the bolts to rotate. The bolts definitely should not rotate within the spring hangers. The bolts' torque settings are not terribly high, around 40 lb. ft. I believe. Provided the wet bolts are well greased, and kept well greased, the bolts shouldn't rotate in the hangers, at that toque setting.
FWIW, 40,000 miles on OEM springs is a lot, and not too surprising to have a spring fail. Knowing what your weight is, and knowing what your springs are rated for, and watching for flattening is a good way to keep from being surprised by suspension problems.