Worth listening to the guys on the podcast on this, although 12 hours is relatively mild: Intermittent Fasting, Lab Testing, Why Sweetspot is Hard & More – Ask a Cycling a Coach 194
I’d suggest your problem is not eating enough before or during the workout. If you don’t, it has a knock-on effect on your appetite the rest of the day, because you’re playing calorie catch-up.
For my Sunday morning 2-hour workout, I had a big bowl of porridge with peanut butter, sultanas, and honey for breakfast, then 2 large bottles of energy mix and 2 energy bars during. It sounds a lot, but it exactly matched the calories expended. And then for lunch and dinner I ate perfectly normal portions of healthy food, and had no issue with hunger for the rest of the day.
And if this 12 hour fast rule means you can’t do this, then I’d lose it, because fuelling workouts properly is way more important in my book.