How "AccountId" is filled from webapi/soap endpoint perspective on Contact entity ?

I have an interesting point, i can't figure out and need your help.
For a client project, i need to integrate a tool focusing on Contacts and Accounts data.

To load a contact, they use the field "_accountid_value" to link contact record with an account record on their side.

I found out that it's a field defined "only" on the Contact view in the database level and is based on this filtering :

Additionnal point i saw, is that as soon as there is a custom field linking the contact entity to account entity, there is an extra LEFT JOIN done in the query but in my example, it's a vanilla environment. 

So far, everything make sense, i got how the field was used from database point of view.

Now my point is : 

• From the DB perspective, no issue, as soon as i create/update a contact record and associate an account via the "ParentCustomerId" field, the "AccountId" field directly gets the new guid of the Account record as you can see below

• From WebApi/Soap endpoint perspective, i have an issue, the result for the same contact record has a null value for the field "_accountid_value", even few hours after the creation.

As you can see below, for the same record value, with the Soap endpoint, the value is "Null" ...

My final questions are : 

Thanks a lot,

Regards,

Clément

PS : today (one/two days later i updated the contact, value is still null)