Hibernate validates collection members removed from collections

I have encountered a strange phenomenon. In a reduced form my entity classes look like this:

@Entity
@Audited
public class Product {

    @Id
    @Column(length = 32)
    private String id = IdGenerator.createId();

    /** Descriptive texts, samples, comments etc.; ONIX composite    Product/OtherText */
    @OneToMany(fetch=FetchType.LAZY, cascade=CascadeType.ALL, orphanRemoval=true)
    @JoinColumn(name="product_id")
    @Index(name="idx_prod_text")
    private Set<Text> texts;

    ...
}

@Entity
@Audited
public class Text {

    @Id
    @Column(length = 32)
    private String id = IdGenerator.createId();

    @NotNull
    @Column(nullable=false, length=3)
    private String type;
    ...
}

HashCode and equals work on the id field. I have built a generic pre-validation routine for auto correction and generation of warnings in a user friendly way. Auto correction for Texts with a null type is removal of this text from the collection in the product ( by calling product.getTexts().removeAll(entitiesToRemove)).

Now I got a product (previously read from DB, so attached to Hibernate session), which gets appended a Text with type null. It is auto corrected, and the Text in question is removed from the collection as described above. So far, so good; inspection of the Product in the debugger shows that the Text is not in the texts collection anymore. But when I call productRepository.saveAndFlush(product) (), I get a

javax.validation.ConstraintViolationException: Validation failed for classes [de.vlx.metadatastore.model.product.Text] during persist time for groups [javax.validation.groups.Default, ]
List of constraint violations:[
    ConstraintViolationImpl{interpolatedMessage='darf nicht null sein', propertyPath=type, rootBeanClass=class de.vlx.metadatastore.model.product.Text, messageTemplate='{javax.validation.constraints.NotNull.message}'}
]

So it seems that this text is still validated by Hibernate? Even if it never was in the data base (which also has of course a NotNull constraint), and is not in the texts collection of the Product anymore?

Even stranger: when I tried to hotfix this by setting a dummy value on the type of the removed entity, I don't get the above exception, but a org.springframework.dao.DataIntegrityViolationException complaining about the fact that type is null.

Which kind of magic is at work, and what can I do to fix that?

Used versions: Hibernate 4.1.7.Final Spring-data-jpa 1.6.0.RELEASE Spring-core 3.2.9.RELEASE