Spock Subjects-Collaborators extension not happy with stubbed method on Spy subject

SettersInjectionOnSpySpec.groovy

package com.blogspot.toomuchcoding.spock.subjcollabs

import com.blogspot.toomuchcoding.spock.subjcollabs.Collaborator
import com.blogspot.toomuchcoding.spock.subjcollabs.Subject
import spock.lang.Specification

class SettersInjectionOnSpySpec extends Specification {

    public static final String TEST_METHOD_1 = "Test method 1"

    @Collaborator
    SomeOtherClass someOtherClassInjected = Mock()

    @Subject
    SomeClass systemUnderTestInjected = Spy()

    SomeOtherClass someOtherClassPlain = Mock()
    SomeClass systemUnderTestPlain = Spy()
    
    def "should inject collaborator into subject without disrupting spy stubbing"() {
        given:
        someOtherClassInjected.getResult() >> 'mock called'
        systemUnderTestInjected.throwException() >> {}

        when:
        def result = systemUnderTestInjected.collaborate()

        then:
        result == 'mock called'
    }

    def "prove that stubbing method on a spy normally works"() {
        given:
        systemUnderTestPlain.collaborator = someOtherClassPlain
        and:
        someOtherClassPlain.getResult() >> 'mock called'
        systemUnderTestPlain.throwException() >> {}
        
        when:
        def result = systemUnderTestPlain.collaborate()

        then:
        result == 'mock called'
    }


    class SomeClass {
        SomeOtherClass collaborator
        
        String collaborate() {
            throwException()
            return collaborator.getResult()
        }

        void throwException() {
            throw new RuntimeException()
        }
    }

    class SomeOtherClass {
        def getResult() {
            return 'SomeOtherClass called'
        }
    }

}