There are numerous bug reports about FileMaker and Outlook. It appears that you've encountered on documented here:
For More Information see: FileMaker 14 64 Bit and Send Mail via Email Client Outlook 64 Bit Not Working
This is one 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