Upgrading buildings is gaining me gold (bug)

I have the weirdest bug. Whenever I finish upgrading a building (no matter what the building is), I instantly gain around 90 gold back. I have no idea why and what triggered it. I can't find any similar bug out there?

Note : this is not an old save, I have started a new game after the recent updates. It also happened to me a long time ago and nothing fixed it besides starting a new game.