@MapsId works different after updating to reactive 3

[majchrzw]

Hello after updating my code to hibernate reactive 3 I'm receiving this error in my code:
org.hibernate.exception.ConstraintViolationException: error executing SQL statement [ERROR: null value in column "book_id" of relation "book_description" violates not-null constraint (23502)] [insert into book_description (description,book_id,language) values ($1,$2,$3)]

My entites look like this:

@Entity
class Book(
    var name: String? = null
) {
    @Id
    @SequenceGenerator(name = "book_seq", sequenceName = "book_seq", allocationSize = 1)
    @GeneratedValue(strategy = SEQUENCE, generator = "book_seq")
    val id: Long? = null

    @OneToMany(mappedBy = "book", cascade = [PERSIST], fetch = LAZY, orphanRemoval = true)
    val descriptions: MutableSet<BookDescription> = mutableSetOf()
}

@Entity
class BookDescription(
    @EmbeddedId
    val id: BookDescriptionId,

    val description: String,

    @ManyToOne(fetch = FetchType.LAZY, cascade = [PERSIST])
    @MapsId("bookId")
    @JoinColumn(name = "book_id")
    var book: Book?
)

@Embeddable
class BookDescriptionId(
    val language: String,
) {
    var bookId: Long? = null
}

And I want to save them:

val book = Book(name = "name")
val description = BookDescription(id = BookDescriptionId(language = "EN"), description = "something", book = book)
book.descriptions.add(description)

sessionFactory.withTransaction { tx -> tx.persist(book) }

I've checked this issue and problem exists in hibernate reactive 3, but everything works fine in hibernate reactive 2 and hibernate 7.
The example code can be found here in 3 branches - https://github.com/majchrzw/mapsid-example.

I've already talked about this on zulipchat but without any real conclusion - https://hibernate.zulipchat.com/#narrow/channel/132096-hibernate-user/topic/.40MapsId.20in.20hibernate.207/with/527690393.

I'm not sure if this is intended behaviour or just a bug, but it stopped working after version update which is strange for me, as I cannot found any note on this in migration guide/release notes for reactive.