GroovyMock misbehave

I will describe a weird (maybe wrong) behavior.

I have three classes:

class Foo {
    String foo

    public Foo(String foo) {
        this.foo = foo
    }
}

class Bar {
    String bar

    public Bar(String bar) {
        this.bar = bar
    }
}

class FooBar {
    private Foo foo
    private Bar bar

    public FooBar() {
        this.foo = new Foo("foo")
        this.bar = new Bar("bar")
    }

    public String printMe() {
        return "${foo.getFoo()} ${bar.getBar()}"
    }
}

I want to Test the FooBar class by catching constructors of dependencies classes Foo and Bar.

import com.scratch.Bar
import com.scratch.Foo
import com.scratch.FooBar
import spock.lang.Specification

import java.util.logging.Logger

public class Mockba extends Specification {
    Logger log = Logger.getLogger("")

    def "foo"() {
        given:

        GroovyMock(Foo, global: true)
        GroovyMock(Bar, global: true)
        Foo foo = GroovyMock(Foo)
        Bar bar = GroovyMock(Bar)

        when:

        FooBar fooBar = new FooBar()
        log.info("${fooBar.printMe()}")

        then:
        new Foo((String)_) >> foo
        new Bar((String)_) >> bar
        foo.getFoo() >> "oof"
        bar.getBar() >> "rab"
        assert 1 == 1 // That's fake
    }
}

The GroovyMock should catch the constructor call over Foo and Bar and supply the mocked version of these objects.

But unfortunately it seems that there is a weird thing that happen because when the instruction:

this.bar = new Bar("bar")

inside the FooBar() constructor is called. The GroovyMock returns the Foo mocked object. The debug watch over new Bar("") gives:

"Mock for type 'Foo' named 'foo'" as Mock result

That's a nonsense for me.

Any suggestion?