Woocommerce customer role doesn’t change if user is already a subscriber

I noticed the following behavior on woocommerce :

I have a user on my website who is already registered with ‘subscriber’ role.

When I login and checkout on my woocommerce shop, user role still remains on ‘subscriber’ and role is not updated to ‘customer’.

Problem is on admin page (Users > All users) : when I use the filter to show all customers, users who were already subscribers and have made orders don’t appear on search results.

I use WordPress 4.9.6 et Woocommerce 3.3.3.

Can anybody explain to me if this behavior is all right ?

Thank you.