From your post it sounds like you are cueing the canter from the trot. I would do it from the walk and see if that makes a difference.
I never give my horses treats as it makes them bite. I had a nieghbor that kept feeding mine apples and I only knew because the horse started biting. They kept denying it until I caught them. The horse quit biting once the treats stopped. If you must give them treats put it in a bucket or on the ground not from your hand. I was at a stable were the owner kept giving her horses carrots and it would snap at you when you walked by it stall that was open at the top half. I told her it was going to bite someone and she said no way would it ever bite someone. Well about an hour later it picked me up off the ground by the shoulder. I could barely drive home and was lucky that I didn't have a serious injury. She now has the top of the stall not open.