It sounds like your ubiquity and your ISP router are on the same LAN segment, which is not a good config.

You should never have multiple DHCP servers configured unless you're intentionally split braining your vlan (only ever done that for HA purposes and using half of the pool on each). Im pretty sure you need to have your ISP connected to your cloud gateway, and all of your gear connected to the ubiquity. Your ISP router should only see your ubiquity, and that's likely a good part of the reason you can't see all the DHCP leases on your ubiquity gear.

Were I in your position, I'd probably disconnect everything and slowly reconnect stuff one piece at a time until you trip over what's causing your issue. I doubt this is the case, but you could also have another DHCP server running on something you forgot about causing issues. Seen that many times before when doing small business network overhauls.