I have the same issue - gateway router losing list of connected devices. I am on the third box since I moved to new place in August of 2016. First two boxes were made by Cisco, this third one is made by Technicolor. I got second box on December 16, 2017, and the third one, by Technicolor (upgraded one), on December 27 2017.
Technicolor box is DPC3941b, manufactured in August of 2017. Worked great for 4 days after it was installed on December 27 2017, and now there is no list of connected devices, and just rebooting router doesn't fix the problem.
Solution from Comcast is to reset gateway to defaults (and I have to reconfiger everything again), or swap for the new box. So, basically, there is no soultion currently.
Is anybody else having the same issue with their leased gateway?