In Ubuntu’s Bionic Beaver release, a change was made to the dpkg version (1.19) where, by default, dpkg-deb enables uniform-compression. This means that the control.tar* file will be compressed using xz by default (or, to be more precise, using the same compression method used on data.tar* files). This means there will be a control.tar.xz file inside the .deb package instead of a more standard control.tar.gz. More information regarding this change is available HERE.