This one's been reported before.
For More Information see: FMPA 11 Drop-Down List Bug 1
This is one of many acknowledged bugs that can be found in the Known Bug List here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
Please note that the "Nuisance" risk level refers to the bugs risk to data and file integrity--not how important a bug it may or may not be.
With that many bugs on the list, I really wish FileMaker would do what Apple did with Snow Leopard:
STOP adding new features and just focus on fixing bugs!
I mean, there's still bugs relating to the "import records" script step going back to the 1980's that still haven't been fixed.
Since neither you nor I can fix the bugs, maybe this enhancement of the basic two column value list, insert ID number value list approach--which avoids the above bug--is worth a look: