I have an Ubiquity USG which serves as a DHCP server, and my Pihole takes care of DNS. I recently changed my inernal domain from 'localdomain' to 'whatever' in the USG, but the GUI for Pihole still shows '.localdomain' behind the name of the clients in the statistics.
I already flushed DNS, I checked the domain name in 01-pihole.conf (shows the new one), I added 'domain=whatever' in that config file, I changed the forwarding of resolution of local names to the new domain name in settings in the GUI, but still, the GUI shows the old one...
Well, I added a new host to my network (a pc that I don't use very much), and it is getting the new domain name in the GUI. It seems like Pi-hole is "remembering" the old domain for clients before the change of the domain name.