If I recall correctly, this was a bug in DL and DA, and it seems to have jumped to Arnor as well: if you have queued a number of buildings on a planet, when one finishes, often, the little green popup says that the building is complete, and that the planet is now building "nothing." However, if you check the planet, it actually is building something, as it's moved on to the next item in the queue. The game does this sometimes, other times, it says the right thing (i.e., factory done, now building farm).