I have the same exact problem. The Xfinity Connect App no longer alerts me when someone calls my land line. And I've also tried toggling notifications in the app, and like you I can see caller ID on my tv, but no longer on my phone. This was a really convenient feature -- knowing for instance if someone was calling my house while I was at work -- but now I only get notified if the person happens to leave my a voice mail on my land line.
Seems to be happening more and more frequently. Ref. one of my replies to a similar thread entitled, "Connect app not receiving calls"