Coop give the right answer. Me I do not see a problem of annealing. I see one coin from 1996 with grease struck. The rest are great. The change in color or shinny? Yes is happened due to the age and environment, but do not degrade the coin. I hate those who say not anymore mint shinny. Ok maybe but you are same like 40 years ago? , no .
For more precision is: The annealing of the Nickels is done at 900 degrees. If it is improper annealing or faster cooling, the color is different. I will look for the color chart for nickel and post here.