I had the same issue you are having but I found out a solution that fixed for me. What ever exotic you get keep it with your character. I got the graviton Lance twice in a row and I decided to keep the second one since I deleted my first and I got a different exotic which was fighting lion. I kept fighting lion and I got six coyote. I kept six coyote and then I got graviton forfeit. Then a few days went by and I got two tailed fox, black talon, and oath keepers.