Since FileMaker Server doesn't have the calculation engine that FileMaker Pro has, FileMaker Server script parameters are actually passed through as text. For example, let's say my script had the following step:
Set Field [Test; Get ( ScriptParameter )]
If my FileMaker Server side parameter was 5 + 5, the field would display 5 + 5 instead of 10.
Aside from creating the single perform script line, you can also capture whether FileMaker Server is running the script and then use Evaluate ( Get (ScriptParameter )).
I will go ahead and submit a feature request for accurate messaging in the FileMaker Server Schedule Assistant and an update to our documentation.
The lack of any logged message in the Server log is espcially "bug like".