lightdm callbacks session-setup-script and session-cleanup-script called at unexpected occasions

I'm using Gnome together with LightDM and am trying to get my background slideshow controlled properly. Unfortunately, this is something several people seem to have tried before, and even with Ubuntu 16.04LTS it seems to be something that is quite difficult to accomplish.

I've added hooks in /etc/lightdm/lightdm.conf for the two events session-setup-script and session-cleanup-script. Now it turns out that session-setup-script is called properly when I first log on with lightdm. When I log out, session-cleanup-script is invoked. However, each time the session is locked (I needed to install light-locker for a secure screen lock for the lightdm/gnome combination which is a story of its own...) and then unlocked again, upon unnnlocking the session-cleanup-script callback hook is invoked, too.

I can't see why this should be the case and consider this a bug. It makes it impossible for me right now to distinguish between an unlock (where at best I'd like to ensure that the desktop background slideshow is running) and a logout scenario where I'd like to stop the slideshow for good.

Any help would be highly appreciated.

Thanks a bunch.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: lightdm 1.18.3-0ubuntu1.1
ProcVersionSignature: Ubuntu 4.4.0-91.114-generic 4.4.76
Uname: Linux 4.4.0-91-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
CurrentDesktop: GNOME
Date: Sun Aug 13 21:39:51 2017
InstallationDate: Installed on 2012-06-28 (1871 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
SourcePackage: lightdm
UpgradeStatus: Upgraded to xenial on 2017-04-24 (110 days ago)