The power bonus to boroughs from Industrialists is lost when the borough is pillaged and doesn't return when it's repaired. Right now in a game I have a city with two boroughs, and two Industrialists in my empire, but the city only has 3 power instead of 4. This is because when I only had one Industrialists, one of the boroughs was pillaged by weather. After repairing it and building the second Industrialists, the city gained 2 power from it as normal, but the one point of power from the first Industrialists, which was in place before the pillage, never returned.