Since at least January, and probably longer, the Xfinity Stream app for Apple TV has been unable to remember your login. You have to login fresh every single time you launch the app.
There are a lot of unhappy users in this thread on the Xfinity forums:
https://forums.xfinity.com/conversations/xfinity-stream-website/xfinity-streaming-service-requires-login-everytime-i-use-it-apple-tv/63ccba40d9aa6753d37a826f
I'm hoping that complaining about it here might catch the attention of some Xfinity devs who can help fix it.