Yes it will generate properly. All the Mistland areas you did not explore will generate as it would if you started anew on a fresh seed. The areas or fringes you did explore will generate Mistland creatures but may not generate Infested Mines and Dvergr component sites. This was my experience on 2 of my maps, I have found plenty of Infested mines and Dvergr component sites. Even the odd Mistland areas I did explore generated islands off shore that had components and mines.