Intermittent fasting causes rapid muscle growth. You should continue to gain weight.
I'd faint, too, if I only ate 1200 calories in a day. Surprised you can make it through the day. Even more surprised your body is managing to lay down new tissue on so little food. It must be very hungry!
Steak, eggs, potatoes - fruits, nuts, berries and forage. Coconut milk and potent herbs and spices. Tea instead of coffee now and teeny amounts of kelp daily. Let's see how this does! Not really had dairy much, and gut seems better for it.