I recently installed Ubuntu 18.04.1 as well as Xubuntu 18.04.1 and both have this issue.
I have a Nvidia GTX 670 for my graphics card for context.
When I installed the OS, one of the first things I did was go to the addational drivers tab and installed the proprietary 390 driver listed and rebooted. After reboot I double checked it was still saying 390 driver, which it was. Cool. But checking in steam it's displaying the Nouveau driver, checked in the terminal and it's saying the same thing.
After that I tried installing it via the PPA here:
And installed the 396 Nvidia driver version first, later the 390. Same results as above, installed just fine but no matter what I do, it wont actually use it.
Next I tried downloading and installing it straight from the Nvidia website. This involved turning off the desktop interface so it wasn't in use during install, but the installer errors out midway through.
The next thing I tried was everything above, but with the Nouveau driver black listed. Nope. If you blacklist the Nouveau driver it shows up as using a "VMWare" driver for graphics.
All of this has been done on both Ubuntu and Xubuntu. I'm at wits end. The last version of Ubuntu I used was 17.10 and installing the prop driver from the additional drivers screen worked perfect.
And today I installed the Ubuntu 18.10 Beta, went to the additional drivers menu and installed the Nvidia 396 Metapackage. After reboot I get a black screen after I type my password in.