Thank you for your post.
I am able to replicate the problem on Mac OS X 10.5.8 and Mac OS X 10.6.4. This works as expected under Windows XP SP3.
I have forwarded your notes along with my findings to our Development and Software Quality Assurance (Testing) departments for review and confirmation. I will keep you informed as information becomes available to me.
Wonder if "under the hood" it's the same as this bug: Sub-menus in FileMaker application menu
Same behavior but different menu.
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
Sounds strikingly similar, only difference is I do get an arrow to show, but no sub-menu pops open. I just grouped the sub-menu items on the parent menu with divider lines separating them from the other menu items. Low-tech workaround...
Our Development and Testing department confirm the problem, as they have grouped this problem with the sub-menu problem under the FileMaker Pro menu (which PhilModJunk has provided the link above). No other information is available at this time.