I installed Lubuntu Bionic Desktop 20180425 under VirtualBox on a Windows 10 host. I tried both with and without 3D acceleration enabled. For me, I didn't get any screen corruption.
However...In the past (over a year ago?) I had exactly the same screen corruption you described.
There has been recent issues with 3D acceleration in VirtualBox. To the point where I turn it off on most of my VMs. Try turning it off to see if that fixes it.
I installed Lubuntu Bionic Desktop 20180425 under VirtualBox on a Windows 10 host. I tried both with and without 3D acceleration enabled. For me, I didn't get any screen corruption.
However...In the past (over a year ago?) I had exactly the same screen corruption you described.
There has been recent issues with 3D acceleration in VirtualBox. To the point where I turn it off on most of my VMs. Try turning it off to see if that fixes it.