User's wallpaper not (always) displayed on login screen

[Emmanuel]

Hi all,
Here is the environnement :
I mount home directory from a NAS. They are mounted as CIFS.
I changed lightdm init script in order to be sure that user's home are mounted before lightdm is started (otherwise things are bad.. very bad).

and the problem i'm facing :

I choose a wallpaper (on the ubuntu set).
I quit my session.
The wallpaper is display when i'm on the user name.
--> This is perfect.
After a while (i use the computer normaly, power on/off, login/logout, ...), lightdm do not display wallpaper for some users but other are ok.
If i continue to use my computer at last, no wallpaper are displayed by lightdm on login screen.
If i set again the wallpaper, everything is back to normal for the user i set.

So, why are the user's wallpaper not display every times by lightdm ?

Please ask for information (with commandline to retrive them), i'll sent it to help solve that pb.

$lsb_release -rd
Description: Ubuntu 16.04.2 LTS
Release: 16.04

$$ apt-cache policy lightdm
lightdm:
  Installé : 1.18.3-0ubuntu1.1
  Candidat : 1.18.3-0ubuntu1.1
 Table de version :
 *** 1.18.3-0ubuntu1.1 500
        500 http://fr.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
        100 /var/lib/dpkg/status
     1.18.1-0ubuntu1 500
        500 http://fr.archive.ubuntu.com/ubuntu xenial/main amd64 Packages

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: lightdm 1.18.3-0ubuntu1.1
ProcVersionSignature: Ubuntu 4.4.0-72.93-generic 4.4.49
Uname: Linux 4.4.0-72-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Apr 8 09:18:43 2017
InstallationDate: Installed on 2013-06-20 (1387 days ago)
InstallationMedia: Ubuntu 12.04.2 LTS "Precise Pangolin" - Release amd64 (20130213)
SourcePackage: lightdm
UpgradeStatus: Upgraded to xenial on 2016-08-14 (236 days ago)
mtime.conffile..etc.init.lightdm.conf: 2016-08-14T20:11:41.830415
mtime.conffile..etc.lightdm.users.conf: 2014-12-13T00:13:38.159029