Seems like the City Details screen isn't getting tidied up like it should when transient effects run their course. One example I encountered is via Aura of Vitality. I cast it on a city and trained some troops, who enjoyed receiving the Constitution bonus to their hit points as they left boot camp to go take on the world. I then dispelled AoV, which saved me the 1 mana per turn tax. City Details confirms AoV has been dispelled but it also still shows the Constitution bonus. I haven't tried training any new troops in that particular city to see if they actually get the CO bonus without AoV being in effect.

I think I've seen the same behavior on other city-boosting spells, but the only hard evidence I've got is regarding AoV.