Thanks for posting.
This issue was previously reported and has been confirmed by our Quality Assurance department. It's since been forwarded to our Development department for further investigation. As a workaround, some users have found that adding a Pause of greater than .6 seconds before the Show Custom Dialog script step can help alleviate the issue.
This has been reported a number of times both here and in the FM GO Forum.
For More Information (and some suggested work arounds), see: Known Buglist for Filemaker GO
This is one of many acknowledged bugs that can be found in the Known Buglist for Filemaker GO 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