In the new 'Bionic Beaver' Ubuntu distribution there is a change in the 'dpkg' version (1.19.0), where 'dpkg-deb' enables '–uniform-compression' by default.
This means 'control.tar*' file will be compressed using 'xz' by default (to be more precise, using the same compression method as 'data.tar*' file).
This claims 'control.tar.xz' file inside the '.deb' file instead of 'control.tar.gz'.
You may find more information regarding this change in this GitHub page.
When building your Debian packages you need to take this change into account.
Artifactory will accept the package even if you do not (and still compress the 'control' file using '.tar.gz'), but installing these packages will not be permitted by the Ubuntu OS itself since it expects the 'control' file inside the package to be with the '.tar.xz' extension and not '.tar.gz'.