JPA - How to get a CREATION_DATE column work

I have a MySQL table like this:

ID_EMAIL  bigint(20)
ID_NEWSLETTER  bigint(20)
SUBSCRIPTION_DATE timestamp  NULLABLE DEFAULT CURRENT_TIMESTAMP

and a JPA class like this:

@Temporal(TemporalType.TIMESTAMP)
@Column(name="SUBSCRIPTION_DATE", updatable = false)
private Date subscriptionDate;

@ManyToOne
@JoinColumn(name="ID_EMAIL",insertable = false, updatable = false)
private AnagraficaIscritti anagraficaIscritti;

//bi-directional many-to-one association to Newsletter
@ManyToOne
@JoinColumn(name="ID_NEWSLETTER", insertable = false, updatable = false)
private Newsletter newsletter;

When I insert an object in other tables the field SUBSCRIPTION_DATE is always updated . This is wrong because I'd like to store only "creation_date" so I'd like to prevent to being updated every time.
So, for example, if I am inserting record 51 all the previous column SUBSCRIPTION_DATE record are updated !!!

|       48 |            34 | 2017-10-11 12:09:42 |
|       49 |            34 | 2017-10-11 12:09:42 |
|       50 |            34 | 2017-10-11 12:09:42 |
|       51 |            34 | 2017-10-11 12:09:42 |
+----------+---------------+---------------------+

Any hints?

the code of the update that could cause the problem should be this:

for (Newsletter item : newsletterItem ) {
boolean isIscritto = false;
for (AnagraficaIscritti iscItem : item.getAnagraficaIscrittis() ) {
    if (iscItem.getIdEmail() == iscritto.getIdEmail())
        isIscritto = true;      
}
if (!isIscritto) {  
    item.getAnagraficaIscrittis().add(iscritto);            
    em.persist(item);           
}

}

I think that when I persist Newsletter object all the column SUBSCRIPTION_DATE are updated at the same value for the same newsletter

I also tried to modify the SQL declaration in

SUBSCRIPTION_DATE timestamp NULLABLE DEFAULT NULL

when it updates the SUBSCRIPTION_DATE updates all the rows to NULL.

Thanks