Why does Powerpoint for Mac not preserve filenames with a slash (/)?
From Microsoft's Known Issues with PowerPoint 2011:


  Some characters cannot be used in file and folder names in PowerPoint
  for Mac 2011
  
  IMPORTANT   This issue is fixed in Office for Mac 2011 Service Pack 1.
  Click Downloads to download Service Pack 1 or the latest update.
  
  Characters, including &lt;"> and |, are allowed in file names in
  PowerPoint 2008 and earlier versions, but they must be removed from a
  file name before it can be opened in PowerPoint 2011.


Check your version of PowerPoint for updates to see if this addresses or fixes the issue.

Otherwise, change the filename to include a character that is allowed such as "-". (e.g.: 9-22)