Just spent ages setting up DHCP reservations on my Superhub 2ac. I did this in order to limit internet access for the kids' devices. I noticed last night that one of their phones had connected to WiFi but the router had given it another IP and so it was not subject to the rules I'd set up. The device was showing up twice in the lease table. I also have another, related gripe in that I cannot back up the settings.
Has your firmware been updated to V2.01.11. (From V1.01.11) by any chance? If so then Backup/Restore in Advanced Settings has been redacted due to Functional problems (no detail from CPE Team afaict if this will corrected in future)
If you haven't tried a FR yet then that may resolve your DHCP Reservation anomaly but if you have the latest V2 f/w then of course you won't be able to restore your customized config from any previous backup!
Forum Team will respond within next 7days and advise whether DHCP Reservation issue is a known problem etc,
Regards Tony "Life is a Binary Inspired Turing Computed Hologram"(don't PM or @Mention me - in case ignoring you offends)