I've also been having a similar issue and cannot find any permanent solution (some folks have reported that a full gateway reset to defaults or setting changes will temporarily resolve the issue but eventually returns to the same behavior). For me, all wireless devices eventually drop off the connected list (using the xFi app) but are still connected to he Internet. The wired devices show up but all previously connected wireless devices eventually are only visible as "previously connected" in the xFi app. Not sure Comcast/Xfinity customer service know any permanent solution (based on the threads I've read out there) and high level tech support may be aware but are not offering a solution in any of the threads or forum conversations I've read. I suspect a real fix may require changes on xfinity's side and I have no idea on the priority or their update schedule. Unless someone from Comcast provides a solution that I have not seen in the many threads I've read, the only thing we can do is complain. I could see this as a big issue for folks needing parental controls - and yes when my kids were younger this was very important to me and that is why I always used a separrate router under my control in the past. Comcast pushed this gateway on me and would not let me use my modem & router with the particular service I have (Gbps with no data cap). It is a nuisance for me since I have no way of knowing who is wirelessly active on services I'm paying.
Update 10/27/2020. After I previously posted, I was in the xFi app and created 3 profiles (the People menu bar selected to create); Home, Management, and Guest and assigned or forgot all previously connected devices to one of these profiles. The devices are showing up again - I also saw that parental controls were available. Not sure if this will continue to work but it has been working for over 12 hours now. No idea why this made a difference or will continue to work as expected.