This is a known issue in FileMaker 13.
For More Information see: Value list set to only show second value bug
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
Thank you for your post.
As PhilModJunk has pointed out, our Testing and Development departments are aware of this issue. I have also attached your post to the original report. When more information becomes available, I will let you know.
I apologize for the late reply. I was reviewing old posts, and realized this issue was fixed in FileMaker Pro 13.0v2. If you have not done so already, please update to the current version (13.0v3) of FileMaker Pro.
Thank you for your reply! updated to 13.0v2 when it came out. I've really enjoyed using FMP 13, especially since the drop down menu fix.