We are using the Mailing List package but we are not using categories with it. I think we have one category created but I don't think we have associated it with anything. I haven't seen your bug.
I don't know if anyone else besides my client and the original client (Greenpeace) have used it yet. My experience with mailing-list is that the basic functionality is great but that there were quite a few bugs to be fixed.
We will try to post all our bug fixes to the bugtracker and I urge you do the same. I'll keep my eyes on it to see if you run into anything we fixed and forgot to report.