(Maybe these steps are obvious but im just asking to make sure)
If you type
$ hostname in one of the nodes, do you get the desired name?
*If not, (again maybe you have done this but it is worth to mention) you maybe have set the hostname with the command
hostname NEW_HOSTNAME. This one sets the hostname until reboot, for a permanent solution, use
$ sudo hostnamectl set-hostname NEW_HOSTNAME
(more info: How to Set or Change System Hostname in Linux)
After this, you should be able to refresh your Netdata Cloud, list your nodes, and see the desired name. The emails use the node’s hostname so this sounds like this issue has something to do with each node’s hostname!
(had the same issue for some of my nodes, and setting it this way fixed it for good)
I hope this is the issue and it is not anything more complicated