Followed your exact steps and did not see the issue. the script I created was in the dialog box as expected when selecting the "specify" button of the perform script options on the button.
Here's my system specs. Did you update filemaker before or after your yosemite update?
I updated the MacBook Air in the following order.
- updated Mac OS X from Lion to Yosemite (10.10)
- updated FileMaker Pro Advanced to 13v3
- updated Yosemite from 10.10 to 10.10.1
- updated FileMaker Pro Advanced 13v3 to 13v4 (the install.log file confirms 10.10.1 was installed at the time of running the FMPA 13v4 installer)
In addition, I attended a FileMaker training course earlier this week and the guy sitting behind me experienced exactly the same issue using FileMaker Pro 13v4 on a two-year old MacBook Pro but I am not sure what version of Mac OS X he was running. At this point in time that makes 2 users who have experienced the issue. Who knows, this could inflate to catastrophic iPhone6 Bendgate proportions (9 users out of the first 10 million sales LOL).
Bye for now.