Hibernate inserts null even when field is not null in the object being saved

I am facing a weird issue where even though all fields are set in the java object, when I save the object hibernate tries to insert null values in the fields.

When I further debugged, I saw that while merging the new entity at this line hibernate generates an empty object and sets to the target instead of setting given entity to the target. This results in insert query with null values.

Am I missing some configuration here? Below are the example entities having associations similar to my case.

class Vehicle {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    @Column(nullable = false)
    @Enumerated(EnumType.STRING)
    @EqualsAndHashCode.Include
    private VehicleType vehicleType;

    @OneToOne(mappedBy="vehicle", fetch=FetchType.LAZY)
    private Car car;

    @OneToOne(mappedBy="vehicle", fetch=FetchType.LAZY)
    private Truck truck;
}

class Car {
    @Id
    private Integer id;
    
    @OneToOne(fetch = FetchType.LAZY, optional = false)
    @MapsId
    @JoinColumn(name = "vehicle_id")
    private Vehicle vehicle;

    ...
}

class Truck {

    @Id
    private Integer id;
    
    @OneToOne(fetch = FetchType.LAZY, optional = false)
    @MapsId
    @JoinColumn(name = "vehicle_id")
    private Vehicle vehicle;

    ...
}