I have the same problem as well. I like to reserve IP addresses for everything on my network, so they don't end up with duplicate IP addresses if I restart the router. Truthfully, I don't know if the Xfinity router has that issue, but I've been assuming it does since that has been true of every other WiFi router that I've use in the past.
Anyway, I replaced a printer, and now I have the old printer still holding an IP address reservation with apparently no way to get rid of it. It's not a big deal at this point, but it's annoying and it's permanent clutter in the offline devices list.
I also tried removing the device and then removing from parental controls, and I also had the result that it just goes back to how it was before I tried deleting it from Offline Devices.
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