I see the same behavior with my Xfinity SG2v2-P. Note that I just got my X1, so I don't have any experience with how it worked before the update. I'm just seeing how the X1 isn't really able to control the cable modem -- even after I "repaired" all the functions by learning from the original remote. In fact that made it worse...

I used menu for power on and home screen (3s) followed by ok for power off. It mimics what I do with the Apple remote. Note that this puts the Apple TV in sleep. HTH