Implicit XWikiAllGroup membership is not supported

[mouhb]

Flash Messages Application (Pro) version : v1.6.1
XWiki version :13.4.3

To reproduce follow these steps :

• Edit the WEB-INF/xwiki.cfg file to enable Implicit XWikiAllGroup membership
  xwiki.authentication.group.allgroupimplicit=1
• Restart tomcat
• Create a new user
• Install the Flash Messages Application (Pro)
• Create a new flash message and set correctly the start and end dates to make it visible
• Login to the wiki using the user created above

Expected result : The flash message is displayed to the user
Current result: The flash message is not displayed to the user, I have to add the user manually to the XWiki.XWikiAllGroup in order to display the flash message