Display settings got lost in the upgrade from 15.04 to 15.10

[Teo]

On my laptop I had my display set up this way, had been so for a very long time:
- whenever I connected an external monitor, the built-in monitor would turn off and the external one would be the only enabled monitor
- obviously when I disconnected the monitor, the built-in one would turn on again

I think the way I had set this up in the first place was by going to Settings->Display with the external monitor connected, turning off the built-in one and leaving only the external one turned on. I don't seem to remember I had to do anything special to have it switch automatically. Which is as it should obviously be: if the built-in monitor is turned off, it must automatically turn on when it becomes the only available one (unless there was a setting whether or not to do so, which doesn't exist).

Whatever the way I had set this up in the first place, the issue is that after upgrading from 15.04 to 15.10 THE SETTING GOT LOST, and both monitors were enabled in a dual-screen mode, an extended desktop or whatever it is called (where both screens are turned on and they are not cloned).

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: ubuntu-release-upgrader-core 1:15.10.14.2
ProcVersionSignature: Ubuntu 4.2.0-23.28-generic 4.2.6
Uname: Linux 4.2.0-23-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Jan 18 21:33:32 2016
InstallationDate: Installed on 2013-10-11 (829 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
Symptom: dist-upgrade
UpgradeStatus: Upgraded to wily on 2016-01-18 (0 days ago)
VarLogDistupgradeTermlog: