Unlearning skills provide a skill learning penalty.
In a discussion of Unlearning skills on the general forum Etna indicated that these skills had been changed to not count towards the learning penalty for other skills.
Yes, that would be a bug. Some more information would be helpful - stuff like, an example of a learning time before you had unlearning, the learning time after you had got the unlearning skill, and how many skills you had total.