I have been looking for a reason to subscribe to this list and I have
now found it. :-)

You are correct it is a bug, but it isn't a Devuan one, Webmin doesn't
know about Devuan, try adding these lines to /usr/share/webmin/os_list.txt

# Devuan
Devuan Linux                    1.0     debian-linux    1.0     `cat 
/etc/devuan_version 2>/dev/null` =~ /^(jessie)/i

It needs to go just above this line

# Debian Linux versions with numbers

Rowland