I had exactly the same bug. I've been charged twice for the exact same interest of my debt...
Last week, I had a negative balance and I've had to pay the interest. This week, my balance was positive before the economical update and I had to ay the same interest I paid a week ago.
Here is a screenshot of my balance where you can see the bug (sorry, it's in French!)
http://hpics.li/4e4c32f