Custom constructors added via metaclass are sometimes not cleaned up

class MetaClassMagicTests extends GroovyTestCase {
    void testIt() {
        ExpandoMetaClass.enableGlobally()
        
        // Child.metaClass // uncomment and everything works
        
        // Save the old meta class
        def oldMetaClass = Parent.metaClass
        
        // Install a new one
        def emc = new ExpandoMetaClass(Parent, true, true)
        emc.initialize()
        GroovySystem.metaClassRegistry.setMetaClass(Parent, emc)
        
        // Install a map based constructor
        emc.constructor = { Map m -> Parent.newInstance() }
        
        // Parent constructor is used, all good
        assert new Child([:]) instanceof Parent
        
        // Reinstate the old meta class
        GroovySystem.metaClassRegistry.removeMetaClass(Parent) 
        GroovySystem.metaClassRegistry.setMetaClass(Parent, oldMetaClass)

        // This fails, this calls the custom constructor from above and returns an instance of Parent
        assert new Child([:]) instanceof Child
    }
    
}

class Parent { def a }
class Child extends Parent { def b }