spockframework/spock

Hello team 😃
I am facing below problem while writing test using SPOCK
I would like to know what knowledge gap I have, to understand why its not working

I have a main class called ClassUnderTest

@AllArgsConstructor
class ClassUnderTest {

    MyCache cache;

    def doSomething(String key, String value){
        cache.add(key, value);
    }

}

A Facade spec called MyFacade.

class MyFacade extends Specification{

    MyCache cache = new MyCache()

    def ClassUnderTest getInstance(){
        return new ClassUnderTest(cache, Mock(OtherDependency))
    }

    def boolean isPresentInCache(String someKey){
        return null != cache.get(someKey)
    }
}

and test spec called ClassUnderTestSpec.

class ClassUnderTestSpec extends Specification{

    MyFacade facade = new MyFacade()

    def 'check if key is added to cache'(){
    given:'get the instance from facade'
    ClassUnderTest instance = facade.getInstance()

    when: ' we call do something'
    instance.doSomething('myKey', 'myValue')

    then: 'cache should have myKey'
    facade.isPresentInCache('myKey')
    }
}

my main goal is to test if ClassUnderTest is populating the cache correctly.
To abstract it out I am using facade. So that my test remains unchanged if the cache class changes say from MyCache to any other class.

So in test I want

1. get the instance
2. pass the key and value to ClassUnderTest
3. check if ClassUnderTest populates the cache using isPresentInCache

Somehow even though facade has cache instance and same MyCache instance is injected in ClassUnderTest, the test doesn't pass 😐️

Thanks for your time and help in advance. 😅
PS: I am a Java developer. Know Groovy only just by writing SPOCK tests