Using latest linux build, I find that often the Nimbatus full screen window gets pushed to the back if I interact with the game's GUI.
I click on something in the game, and the game window is suddenly in the back, with my other apps being popped to the front.
$ cat /etc/issue
Ubuntu 18.04.1 LTS \n \l
This can happen a few times, and I think it stops as I minimize all other apps.
Player.log