You didn't mention your location.
Rust is a possibility, but as I understand it, it is caused by rain and humidity, and generally an issue in the south. If that is the situation, then treatment is definitely the correct course of action.
Could be underwatering and/or overheating with low humidity are some point in the past. As long as new growth is healthy, past water issues are not an issue now. If figs get stressed, they will shed leaves, but the plant will be fine. We had 100+ temps a week ago, with 10% humidity. Trees shed leaves, had yellowing leaves, brown spotted leaves, crispied leaves all over the place, but they are fine.