The tranny fluid would be lower than the seam, so while sitting still it wont leak, but when getting thrown around while spinning it is being splashed up to the seam. Make sure the bolts in the area are all tight. There is no gasket in the seam, only a thin bead of sealant. Sounds like maybe there's a spot that was missed. If you clean it right off with brake cleaner or contact cleaner, and then put a little sealant on the outside it may hold. (let it dry completely before driving again). If not, split the tranny or you could live with a little anti rust action.