A neutral feeling has the inherent quality of being Dukkha because it can make us stupid. When we don't have any feeling at all we might become numb, we might become indifferent, and in that way it has the inherent quality of being unsatisfactory. Now, we should point out that this seems to cover all Vedana as Dukkha, unpleasant, pleasant, and neutral, but let's remember that there is spiritual and non-spiritual Vedana. So if a person is actually having some spiritual pleasant feeling based on the fact that they just became enlightened, I don't think that one is Dukkha.