Hibernate inserting NULL values into @ManyToOne @JoinColumn when given field is clearly set

Question

I've got a weird issue here that I've been debugging for 2 days now and which I can't get my head around.

Basically I've got two entities: Company and EkasaCashRegister.
The EkasaCashRegister entity references Company entity via a @ManyToOne mapping.
Both those entities have tax_id column in the database and it's declared using @JoinColumn annotation.

The problem is that whenever I try to persist the EkasaCashRegister entity, even though it has the company field set to a managed instance of Company entity, Hibernate attempts to write NULL into the tax_id column of the ekasa_cash_registers table.

Entities look like this:

@Entity
@Table(name = "companies")
public class Company
{
    @NotNull
    @EmbeddedId
    private CompanyIdentity identity;

    @NotBlank
    @Size(max = 15)
    @Column(name = "tax_id", nullable = false, updatable = false, unique = true, length = 15)
    private String taxId;

    // other fields and boilerplate code omitted

    @Override
    public String toString()
    {
        return "Company{" +
                "identity=" + identity +
                ", taxId='" + taxId + '\'' +
                '}';
    }
}

@Embeddable
public class CompanyIdentity implements Serializable
{
    @NotBlank
    @Column(name = "company_country_code", nullable = false, updatable = false, length = 2)
    protected String companyCountryCode;

    @NotBlank
    @Column(name = "company_registration_number", nullable = false, updatable = false, length = 15)
    protected String companyRegistrationNumber;

    // boilerplate code omitted

    @Override
    public String toString()
    {
        return "CompanyIdentity{" +
                "companyCountryCode='" + companyCountryCode + '\'' +
                ", companyRegistrationNumber='" + companyRegistrationNumber + '\'' +
                '}';
    }
}

@Entity
@Table(name = "ekasa_cash_registers")
public class EkasaCashRegister
{
    @Id
    @NotNull
    @Column(name = "cash_register_code")
    private Long cashRegisterCode;

    @NotNull
    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "tax_id", referencedColumnName = "tax_id", nullable = false, updatable = false)
    @OnDelete(action = OnDeleteAction.CASCADE)
    private Company company;

    // other fields and boilerplate code omitted

    public EkasaCashRegister(Company company, Long cashRegisterCode)
    {
        Objects.requireNonNull(company, "company=null");
        Objects.requireNonNull(cashRegisterCode, "cashRegisterCode=null");

        this.company = company;
        this.cashRegisterCode = cashRegisterCode;
    }

    @Override
    public String toString()
    {
        return "EkasaCashRegister{" +
                "cashRegisterCode=" + cashRegisterCode +
                ", company=" + company +
                '}';
    }
}

Now here is the actual code used to persist the EkasaCashRegister entity:

var newCashRegister = new EkasaCashRegister(
        pos.getBranch().getCompany(), // returns managed instance of Company from @ManyToOne mapping on Branch
        cashRegisterCode
);

// setters for some further attributes have been omitted

System.out.println("newCashRegister:");
System.out.println(newCashRegister.toString());

em.persist(newCashRegister);

Which outputs:

newCashRegister:
EkasaCashRegister{cashRegisterCode=88812345678900001, company=Company{identity=CompanyIdentity{companyCountryCode='SK', companyRegistrationNumber='36684449'}, taxId='2022251528'}}
17:57:12.999 DEBUG SQL                                      : 
    insert 
    into
        ekasa_cash_registers
        (created_by, created_date, last_modified_by, last_modified_date, auth_data, auth_data_exp, auth_data_pass, tax_id, id_data, cash_register_code) 
    values
        (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
17:57:12.999 TRACE BasicBinder                              : binding parameter [1] as [VARCHAR] - [root]
17:57:12.999 TRACE BasicBinder                              : binding parameter [2] as [TIMESTAMP] - [2022-04-05T15:57:12.954669Z]
17:57:12.999 TRACE BasicBinder                              : binding parameter [3] as [VARCHAR] - [root]
17:57:12.999 TRACE BasicBinder                              : binding parameter [4] as [TIMESTAMP] - [2022-04-05T15:57:12.954669Z]
17:57:13.004 TRACE BasicBinder                              : binding parameter [5] as [OTHER] - [*redacted:authData*]
17:57:13.007 TRACE BasicBinder                              : binding parameter [6] as [TIMESTAMP] - [2023-03-16T09:28:42Z]
17:57:13.007 TRACE BasicBinder                              : binding parameter [7] as [VARCHAR] - [*redacted:identityData*]
17:57:13.007 TRACE BasicBinder                              : binding parameter [8] as [VARCHAR] - [null]
17:57:13.064 TRACE BasicBinder                              : binding parameter [9] as [OTHER] - [*redacted:authDataPass*]
17:57:13.066 TRACE BasicBinder                              : binding parameter [10] as [BIGINT] - [88812345678900001]
17:57:13.071 WARN  SqlExceptionHelper                       : SQL Error: 0, SQLState: 23502
17:57:13.071 ERROR SqlExceptionHelper                       : ERROR: null value in column "tax_id" of relation "ekasa_cash_registers" violates not-null constraint
  Detail: Failing row contains (88812345678900001, null, *redacted:identityData*, *redacted:authData*, *redacted:authDataPass*, 2023-03-16 10:28:42, root, 2022-04-05 17:57:12.954669, root, 2022-04-05 17:57:12.954669).

Of which the following is important:

1. EkasaCashRegister#company#taxId is clearly NOT NULL:

EkasaCashRegister{cashRegisterCode=88812345678900001, company=Company{identity=CompanyIdentity{companyCountryCode='SK', companyRegistrationNumber='36684449'}, taxId='2022251528'}}

2. Yet Hibernate binds the tax_id as NULL:

17:57:13.007 TRACE BasicBinder                              : binding parameter [8] as [VARCHAR] - [null]

3. And then it complains the database rejected that NULL value

ERROR: null value in column "tax_id" of relation "ekasa_cash_registers" violates not-null constraint

I would be really glad if someone could advise me on what I'm doing wrong here.
Thanks in advance.