I was having a similar problem with one of my Northern Mist bows, when I asked Steve about why it was happening with just this bow, (I have 5 different models of his bows, I know I may have a problem) we happened to be at the bow show in Kalamazoo, he had me hold and draw the same model I was having trouble with, my form was ok but he had me loosen my grip slightly with my index and middle finger, and tighten it with my ring finger and pinky, and not surprising it worked no more arm slap. I still wear an arm guard just because I have worn one for years, and like you I shot recurve for years before switching to Hill style bows about 5-6 years ago, now I can go back and forth between the different styles without any issues. Good luck and happy shooting.
Jason