The first time it happened to me, I thought maybe I had fulfilled the mandate too close to the election, but now I'm sure this is a bug. Mandates to bring surplus of a given resource above a given level seem to never register as being fulfilled. Most recently, I had a mandate in a new game to bring mineral surplus above +20. I accomplished this by mid-term, and when the election was announced I had a solid +23, but after the election was held the game told me my mandate was unfulfilled. This strikes me as nearly game-breaking, given that I haven't been able to successfully complete any such mandate in my ~40 hours of gameplay. All other mandates seem to be working normally, incidentally.