https://www.debian.org/doc/manuals/maint-guide/dother.de.html#conffiles

=> The command to install the debian package (which automatically configures the conf files) was missing.
      It autoselects all files in /etc as config file so conffiles can be empty (but must still exists).

man dh_installdeb
       package.conffiles
           This control file will be installed into the DEBIAN directory.

           In v3 compatibility mode and higher, all files in the etc/ directory in a package will automatically be flagged as conffiles by this program, so there is no
           need to list them manually here.
Merge pull request #474 from Reiner030/debian-conffiles-installfix

https://www.debian.org/doc/manuals/maint-guide/dother.de.html#conffiles