The message "Ignoring domain 5G-5 for DHCP host name USW-Flex-2" is appearing under Pi-hole Diagnosis in my Pihole admin console. The host in question is a Unifi 5-port 2.5GBE switch. In the Unifi network controller software it shows with the name "USW Flex 2.5G 5". When I look at DHCP leases in the Pihole admin console, the switch shows with the host name "USW-Flex-2". So it looks as though Pihole is taking the part of the switch name up to the period as the host name and the part after the period as a domain name.
Try naming your own Windows PC something similar and see what happens : Usually you either get a nice error or Windows just converts it to something similar with a small correction 
It's a very easy way to check if the hostname you were planning to use for something else is correct or not...
Periods are used as delimiters for hostnames. They are not valid inside a hostname, they will be split in to two fields.
OK, I used the Unifi network controller to replace the period in the switch name with a hyphen, and Pihole now sees the full switch name and the error message has disappeared from the Pi-hole Diagnosis page.