I can confirm this issue. Through testing, here is what I discovered:
The issue happens when the "Body" is empty. If you place any character into the body and then run the script then it will work.
For example, I have added the text <<insert body text here>> into the SEND Mail script. Then the email will pop up. You can also do it by adding a simple "." Basically anything.
It is not ideal - but at least it seems to get it to pop up for additional editing.
I hope this helps - but I also hope to see this resolved in a future update.
For Tech Support: I have update Outlook to Version 14.2.2. (140509) in hopes that things would improve, but no luck.
There are documented issues with OutLook and FileMaker, but the_9th's post describes something different. I suggest posting an issue report documenting this behavior in Report an Issue in order to bring it to the attention of TS personnel that work for FileMaker Inc. (This part of the forum is 99% "self help" so it doesn't get the frequent TS replies that you see in that section.)
For More Information on documented issues with FMP and Outlook see: Send Mail using Outlook 2010 Error
These are two 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