My M8 was doing the same thing, except it would usually go back into foward. I took it to arctic cat last week and they ended up cleaning a switch/sensor(I didn't quite understand what he was talking about) on the side of the chian case. I rode it last weekend and used the reverse a couple times and it shifted good every time.
I did notice that if the sled was really good and warmed up or if it was in the garage if would shift without the ccu code. So maybe if its possible you could get it in a warm garage or shop and then it might shift.