Well, it clearly says you are in positive income. My first guess would have been, that you somehow payed more than you had stored, but if you say, it has been going down for several turns, that is unlikely. Could you share a save file?
I am well past that point in time now and I have worked my way back to being positive, how I do not know, but that is the point. There needs to be a way of accounting for Control just like there is for income, trade revenue, and all resources.