When you try to edit subbeans using a composite field it has not effect for bean's attributes that are not initialized (i.e. attribute = null).

It seems to be related to this code in info.magnolia.ui.form.field.AbstractCustomMultiField.createLocalField(FieldDefinition, Property<?>, boolean):

 // If the value property is not an Item but a property, set this property as datasource to the field
// and add a value change listener in order to propagate changes
        if (!holdsItem(property)) {
            if (property != null && property.getValue() != null) {
                field.setPropertyDataSource(property);
            }
            field.addValueChangeListener(selectionListener);
        }

When the value of the bean attribute is null, setPropertyDataSource is not set to property. So for the attributes that are not null have set correctly the datasource method pointing to the Bean and can be edited, but the attributes that are null are pointing to a transformer and edition has not effect.