Steps to reproduce the issue

In the front-end, modules assigned to the Guest group. However, in the back-end, when a module is assigned to the Guest group, they cannot longer be accessed from the list by the Super User.

To test it, in a clean Joomla 3.8.2, assign a module ("Most Read Posts" or "Older Posts") to the Guest group.

Expected result

The modules must be shown in the module manager always to the administrator (Super User).

Actual result

After it is assigned to the group, the module disappears.

System information (as much as possible)

J 3.8.2

Additional comments

The issue seems to be located here:

administrator/components/com_modules/models/modules.php (line 317-319)

$user   = JFactory::getUser();
$groups = implode(',', $user->getAuthorisedViewLevels());
$query->where('a.access IN (' . $groups . ')');

If the administrator user is manually added to the Guest group, then the admin user can access the Guest modules.

Issue found and credit to |@sergiois