There is a bug with the Preview button that causes content to disappear from its group content list and changes its display.
The content is still in the system, but has lost its context (it no longer belongs to a group) and therefore is un-editable by users within that group. This occurs when you click Preview, then click Save. The content is saved at this point, but the connection to the group is lost and the node will not appear in the group's Content List. Because the content still exists, it can still be accessed if the URL is known, but the display for the content is abnormal because it has no group to exist within.
Resolution: Until the bug can be fixed, the Preview button has been removed to avoid further loss of content.