I do not believe that it will accept comma-separated entries like that as a valid value - you probably need something in between the selection and the attribute setting code to take the comma separated list and process it through a loop.You might be able to see what's happening a bit more if you enable workflow debugging mode.
The good news is that iw_solution_email.ipl is a Perl script - if you know anything about perl you should know how to add debugging code to find out what is getting passed.