Mock assigned to instance variable is null during Spock Groovy test

I am encountering a strange error with Spock when unit testing a Groovy class. In the test I am mocking a Jenkins CI class (not sure if relevant). During my test the ctx instance variable of MyClass is null so I get a NullPointerException, even though it is not null when created in the test ("when" feature).

Am I doing something wrong here? It appears that the mock object reference is cleared during the test. Below is a simplified example to illustrate:

import org.jenkinsci.plugins.workflow.cps.CpsScript
import spock.lang.*

class MySpec extends Specification {

    def 'does something'() {
        given: 'valid input'
        def foo = 'foo'
       
        when: 'something happens'
        def context = createContext()
        assert context
        def obj = new MyClass(createContext())
        def success = obj.doSomething(foo)
        
        then: 'it is successful'
        assert success
    }

    private def createContext() {
        def ctx = Mock(CpsScript)
        ctx.sh(_) >> "sh called"
        return ctx
    }
}

class MyClass {

    private final def ctx

    MyClass(ctx) {
        this.ctx = ctx
    }

    def doSomething(foo) {
        ctx.sh('// bash script that uses foo')
    }
}