abstract class SuperBuild {
    // If I comment this out, the problem goes away
    abstract String getTestString()
}

class Build extends SuperBuild {
    String testString = 'hello'

    // Replace above with this and it works
    //String getTestString() { 'hello' }

    // Adding @NonCPS annotation here also makes the problem go away
    void test() {
        // Works as expected
        assert testString == 'hello'

        Closure cl = {
            // Works as expected
            assert resolveStrategy == Closure.OWNER_FIRST
            assert getOwner() instanceof Build
            assert this.testString == 'hello'

            // Fails with: No such field found: field org.jenkinsci.plugins.workflow.cps.CpsClosure2 testString
            assert testString == 'hello'
        }()
    }
}

new Build().test()