This is a known issue with FileMaker 12.
For More Information see: http://forums.filemaker.com/posts/ea146d9cec?start=1&stop=10#189429
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
To futur visitors :
I performed new tests about this issue with FileMaker Pro 12.0v4 on both Mac OS X 10.8.3 and Windows 7 systems and the bug seem to has been fixed.
Well done FMI !!!