When changing the hostname on Debian 9 the hosts file is not updated. There is an entry in /etc/hosts for the 127.0.1.1 ip number:
127.0.1.1 name.domainname.com name
This is correctly set when first installing the server, but not updated when changing the hostname from webmin. I haven't found any impact on my server, but is probably best to make sure the hostname in in the hosts file is updated correctly.
See also Debian documentation: https://www.debian.org/doc/manuals/debian-reference/ch05.en.html#_the_ho...