It sounds to me like you may be torquing the string, which will make it react unpredictably. I would expect the callous to form primarily on the middle finger for a split finger shooter. If you don't employ enough back tension, though, you can end up drawing with your arm muscles (biceps are not the right muscles to use) and employ too many hand muscles, causing you to "manhandle" the string, twisting it as you draw. This leads to callouses on the third finger and on the side of the index finger and will also cause the arrow to pop off your rest or shelf inadvertently while drawing.