This issue can be reproduced in version 70.0.3538.76. But only if the "User data" policy is set to "Erase all local user data".

Steps to reproduce the problem:
1. Go to home screen
2. Touch mouse pad with 2 fingers
3. Select wallpaper app
4. Click in the option to close the window in the top right corner

What is the expected behavior?
The wallpaper is defined.

What went wrong?
As soon as the option to close the window is selected the wallpaper will return to the default one for the device.

Workaround: Since this problem is happening only when the "User Data" policy is configured, you can follow the steps listed below:

1) Sign in to the Admin console
2) Go to Device management
3) Click in Chrome management
4) Go to Device settings
5) Select the organizational unit where the device is located
6) Look for the policy "User Data" and change the value to "Do not erase all local user data"
7) Save the changes

#17524965

This issue has been reported by one of our Enterprise/EDU customers.
Could you have a look at it please?

Thank you

Logs/policies: https://drive.google.com/drive/folders/1ylTYRE3pCUjMgLzOli_N4MLEnsVWc1qV?usp=sharing

1) It's intended that the wallpaper should be reverted to default after user logs out, if the "User data" policy is set to "Erase all local user data".

2) The bug in #2 is about that the wallpaper is reverted to default too early, when user closes the wallpaper app? Is the understanding correct?

3) The original bug description and title are not very clear. Not sure if it's the same issue with #2.

Here's a video reproducing the issue
 
https://drive.google.com/file/d/1tdoS2aqUo15uqAhsJNijDYCkkBTyRs1z/view?usp=sharing 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f5402766718b22a0251b67f50fa4ee7654fcc6f3

commit f5402766718b22a0251b67f50fa4ee7654fcc6f3
Author: Wenzhao Zang <wzang@chromium.org>
Date: Tue Nov 20 23:16:44 2018

cros: Ephemeral user's wallpaper should be kept within the session

When ephemeral user selects a new wallpaper, it's not saved to disk.
But we should still be able to keep this wallpaper before user logout.
The wallpaper is already cached in memory, but we ignored the cache for
ephemeral users: so when lock screen or wallpaper app calls
ShowUserWallpaper, it always thinks that ephemeral users never set a
wallpaper and then display the default wallpaper.

Bug: 825237,  902327 
Change-Id: Id9a811c91243b04c7459f3ee20569ec4fdff14b0
Reviewed-on: https://chromium-review.googlesource.com/c/1343105
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609844}
[modify] https://crrev.com/f5402766718b22a0251b67f50fa4ee7654fcc6f3/ash/wallpaper/wallpaper_controller.cc
[modify] https://crrev.com/f5402766718b22a0251b67f50fa4ee7654fcc6f3/ash/wallpaper/wallpaper_controller_unittest.cc

The issue in #2 and # 7 is fixed.

In terms of the bug title, the "+ button" is intentionally removed.

I am having the same issue when users are trying to set a background when "erase local user data" is set in policy. The background chnages but when the selector is closed, the wallpaper reverts back.

Hi, the issue is fixed in 72.0.3617.0.