Groovy closure returning strange value when called for a mock

I'm unit testing a file I wrote, and there's a particular service that I'm mocking out which is turning out to be quite problematic.

This should be minimum readable code

EmailSpec.groovy

void "throwaway test name"()
{
   given:
   def problematicServiceMock = mockFor(ProblematicService)

   //Used for configurations.  In this case, there are no parameters since
   //individual parameters should be null safe.  Might need to initialize
   //params for problematicService to false to avoid null issues.
   def params = new GrailsParameterMap([:], null)
   //in test code, this has values and is a shared item, but none of those 
   //should be important here
   def user = new User()

   //Simple return just to imitate functionality
   //If I change arguments list to match arguments from source service, get
   //complaint about no arguments matching emptylist, User, null, null, null
   //The below format worked in another test
   problematicServiceMock.demand.sampleFunction (0..1) {List departments,
                                                        User user ->
   Set<String> sampleOutput = ["sample output"]
   sampleOutput
   }

   service.problematicService = problematicServiceMock.createMock()

   when:
      def myMap = service.getMyMap(params, user)
      Set results = myMap.getEmailList()

   then:
      results.size() == 1
      println(results) //manual checking in place of assert temporarily
}

EmailService.groovy

  //...

  Map getMyMap(GrailsParameterMap params, User user) {
  //Bunch of irrelevent processing code to populate map
      Map myMapData = [:]
      myMapData.emails = [
                        //Bunch of other services that are retrieving fine
                        //Sorry for arm-length arguments
                        sampleMap: problematicService.sampleFunction(
                                                  params.list('filler'), 
                                                  user, 
                                                  params.boolean('filler2'), 
                                                  params.boolean('filler3'), 
                                                  params.boolean('filler4'))

   myMapData.getEmailList = { ->
      Set<String> emailTo = [] as Set
      myMapData.emails.each {  k,  v -> //this line previously had String k, Set v, but 
                                        // whined about converting closure to set.
                                        //Definitely a hint to the problem
      emailTo.addAll(v)
      }
      emailTo
    }
 myMapData
 }

problematicService.groovy

class ProblematicService {
//irrelevant

Set<String> sampleFunction(List<String> listArg, User user, boolean boolArg1, boolean boolArg2, boolean boolArg3) {
//irrelevant since it's being mocked out
   }
}

Instead of outputting [sample output], I get a reference to the closure's anonymous identity. myPackage.EmailSpec$_$spock_feature_0_5_closure7@6c9cd7aa This obviously isn't correct. There are three other services that are returning just fine, but they don't have the primitive arguments and only consist of a List (shows empty in testing) and user. Not sure what to do here. The problem is apparent, but not the solution.