My Omnia offers to clients free IP addresses from DHCP pool also for devices, which has reserved IP address. E.g. my laptop’s wifi has reservation to x.x.x.31, but Omnia sent it x.x.x.143.
I checked, that Omnia is the only DHCP server in a network, also DHCP client log says, that it’s Omnia (by the DHCP server’s IP). Also MAC address was checked and it’s the same in reservation and in current leases list.
What exactly is youre the settings DHCP and DNS in Luci - there is no duplicity (two MAC … one IP) ? Problem arose immediately after setting fixed IP or after some time of correct operation? Router is not connected to the gust network 
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… my experience with dynamic/static lease…
I faced this issue with my “TV”, dynamic lease was not working (resp. was, but not my wanted IP). If you play with dhcp/dns and lease time, you have to sometime wait until that lease/ip is released(free). In mean time if you get new one, DHCP will use that one next time (not that previous one). Since some time i do not care which ip it is, if that ip is same as last time.
You can play with “lease” files (and remove correspond entry, so no need to wait).
If you set static lease (or use hosts/ethers), you have to remove entries from related files before you switch back to dynamic. Otherwise you never know which IP will be preferred.
Regarding wifi, in some cases (intel adapters), you can benefit from dual-interface. (in my case i can have three different MAC on my laptop wifi adapter – depending if i let Intel-App to manage it or Windows default ) … anyway no matter which MAC you should be able to get same IP over and over again. (do you have correctly set hostname in luci for each adapter(MAC) address ?) …
Also do you have local domain set on? If so, is that laptop member of that domain or is it “work” laptop and is registered to company domain? (that might play role as well).
I have a range of allowed IP addresses 101-120. For some unused as a reserve I set a non - existent MAC address. There is no free IP for dynamic allocation. Therefore, I must always enable the new client by typing his MAC on the reserve IP (1x yearly ). Random guests logs in to the guest WiFi.