Overriding final of AtomicBoolean type doesn't work

[TheFriendlyCoder]

I have been experimenting with Spock lately as a potential replacement for JUnit / Mockito in some unit tests I am working on and have been struggling with mocking of classes that have final fields and methods. I was hoping this Spock plugin might address at least some of my problems, but I can not get a very simple POC Spock test to work properly. The simplest example test I've found is shown below:

import java.util.concurrent.atomic.AtomicBoolean

@Mockable(AtomicBoolean)
class SampleTestSpec extends Specification {
    def "Test Final Mock" () {
        given:
        AtomicBoolean activeFlag = Mock(AtomicBoolean.class)
        activeFlag.get() >> [true, false]

        expect:
        // First true
        activeFlag.get()
        // Then false
        !activeFlag.get()
    }
}

I am quite new to Java and Spock so I am wondering if I am just doing something fundamentally wrong here, or if this is perhaps a bug with this plugin?