ComboBox - setItemCaptionMode does not work as expected

Hi everybody,
at the moment, I'm struggling with modelling a 1:n relationship with combo boxes: I have an entitiy "Nutrient" and an entity "NutrientUnit". Each Nutrient can have a NutrientUnit. When I edit the Nutrient, I want to select the NutrientUnit by a combo box. Basically this works fine.
If I try to use setItemCaptionPropertyId, the list in the combo box shows the correct representation from the property I chose. Merely the combobox is not prefilled anymore and stays empty.
Furthermore, if I do not use setItemCaptionPropertyId, the 'toString()' representation is shown (so combobox is filled).
Everybody else like loading and saving to DB works fine.
Can anybody help here ? I tried to reduce everything to an example:

package calculator.ui.view.impl;

import javax.annotation.PostConstruct;
import javax.enterprise.context.SessionScoped;
import javax.inject.Inject;

import calculator.model.db.entities.impl.Nutrient;
import calculator.model.db.entities.impl.NutrientUnit;
import calculator.presenter.impl.NutrientPresenter;
import com.vaadin.data.fieldgroup.BeanFieldGroup;
import com.vaadin.data.fieldgroup.FieldGroup.CommitException;
import com.vaadin.data.util.BeanItem;
import com.vaadin.data.util.BeanItemContainer;
import com.vaadin.ui.AbstractSelect.ItemCaptionMode;
import com.vaadin.ui.Button;
import com.vaadin.ui.ComboBox;
import com.vaadin.ui.CustomComponent;
import com.vaadin.ui.FormLayout;
import com.vaadin.ui.TextField;

@SessionScoped
@SuppressWarnings("serial")
public class NutrientDetailForm extends CustomComponent {

    @Inject
    NutrientPresenter presenter;

    FormLayout formLayout;

    BeanFieldGroup<Nutrient> beanFieldGroup;
    BeanItemContainer<NutrientUnit> nutrientUnitContainer;
    BeanItem<Nutrient> nutrient = null;

    TextField name;
    ComboBox nutrientUnit;

    Integer id = new Integer(1);

    @PostConstruct
    public void init() {
        formLayout = new FormLayout();
        name = new TextField("Name");
        nutrientUnit = new ComboBox("Unit");
        nutrientUnit.setWidth("1500px");
        formLayout.addComponents(name, nutrientUnit);

        // create BeanFieldGroup and bind fields
        beanFieldGroup = new BeanFieldGroup<Nutrient>(Nutrient.class);
        beanFieldGroup.bindMemberFields(this);

        // create combo box with all nutrient units
        nutrientUnitContainer = new BeanItemContainer<NutrientUnit>(NutrientUnit.class, presenter.getNutrientUnits());
        nutrientUnit.setContainerDataSource(nutrientUnitContainer);
        nutrientUnit.setItemCaptionMode(ItemCaptionMode.PROPERTY);
        nutrientUnit.setItemCaptionPropertyId("unitName");
        nutrientUnit.setImmediate(true);

        // button to switch between 2 instances of Nutrient (id 1 or 2)
        Button b = new Button("Test");
        b.addClickListener(e -> loadExample());

        Button s = new Button("Save");
        s.addClickListener(e -> saveEntity());
        formLayout.addComponents(s, b);
        setCompositionRoot(formLayout);

    }

    private void saveEntity() {
        try {
            beanFieldGroup.commit();
            presenter.save(nutrient.getBean());
        } catch (CommitException e) {
            e.printStackTrace();
        }

    }

    // switches between Nutrient 1 and 2
    public void loadExample() {
        id++;
        nutrient = new BeanItem<Nutrient>(presenter.getEntityById((id % 2) + 1));
        beanFieldGroup.setItemDataSource(nutrient);
        try {
            beanFieldGroup.commit();
        } catch (CommitException e) {
            e.printStackTrace();
        }
    }
}

package calculator.model.db.entities.impl;

import java.util.List;

import javax.persistence.Entity;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;

import calculator.model.db.entities.abstr.AEntity;

@SuppressWarnings("serial")
@Entity
public class Nutrient extends AEntity {

    String name;

    @OneToMany(mappedBy="nutrient")
    List<NutrientValue> nutrientValues;
    
    @ManyToOne
    NutrientUnit nutrientUnit;

    ... /* setters & getters */
    
//seems not to be considered
    @Override
    public String toString() {
        return this.name;
    }

}