Steps to reproduce the issue

• Activate actions logging in Joomla 3.9.1.
• Set error reporting "Maximum".
• Install extension EJB - Easy Joomla Backup of @Kubik-Rubik
  https://extensions.joomla.org/extensions/extension/access-a-security/site-security/easy-joomla-backup/
• Go to User Actions Log overview

Actual result

• Because the plugin name is plg_easyjoomlabackupcronjob and not plg_system_easyjoomlabackupcronjob $parts[2] is not defined
  https://github.com/joomla/joomla-cms/blob/3.9.0/administrator/components/com_actionlogs/helpers/actionlogs.php#L116

Question now is

Should Joomla core change something or is it a must for developers in the future to name plugins correctly?

if (count($parts) > 2)
{
 $source = JPATH_PLUGINS . '/' . $parts[1] . '/' . $parts[2];
}
else
{
 $source = '';
}