Ubuntu 18.04 installs by default will not preserve the hostname unless specifically set in /etc/cloud/cloud.cfg to preserve hostname.
To fix this, modify the /etc/cloud/cloud.cfg file and change the "preserve_hostname" parameter from "false" to "true" - then edit the /etc/hostname file to your desired hostname and reboot your server.
After rebooting, you will notice that your new hostname is preserved and not set to "ubuntu"
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