Shopping Cart Basket Not Persisting Data Accross Sessions & Login Aspects

[Virgoat]

The shopping cart should persist from visit to visit and between logins and logouts. It should not require that logins are necessary. The cookies should provide enough information to synchronize offline activities with logged-in activities.
This is normal to all websites. gnc.com, amazon.com, webstaurantstore.com, etc.

Prerequisite- Cookies ARE enabled.

Use-case-scenario for shopping cart feature, problem identified: temporary cart not adding items to existing cart under a user profile.

Instructions to repeat problem provided below:

1. existing user visits website but is NOT signed in
2. existing user browses
3. existing user adds items to shopping cart (temporary cart)
4. existing user clicks 'Check out'
5. existing user is directed to 'Shopping-Cart Summary' page
6. existing user clicks 'Proceed to checkout'
7. existing user is directed to 'Sign in' page
8. existing user signs in under the ALREADY REGISTERED? area.
9. existing users items are immediately saved in their shopping cart under their customer profile.
10. existing user does NOT finish checkout process
11. existing user clicks 'Sign out'. (at this point, the user may close the window, or leave it open, it doesn't matter, the point is, the user has SIGNED OUT of their account with the website.)
12. existing user returns to the website and is NOT signed in.
13. existing user adds items to the [temporary] shopping cart. They are still NOT signed in yet. Existing user is aware they already have other items saved in their registered profile.
14. existing user clicks 'Check out'
15. existing user is directed to 'Shopping-Cart Summary' page
16. existing user clicks 'Proceed to checkout'
17. existing user is directed to 'Sign in' page
18. existing user signs in under the ALREADY REGISTERED? area.
19. existing user's original shopping cart from the first sign-in are removed and replaced with the new items from the second shopping cart.
20. existing user is furious that they have to now search for the the other items that were previously saved in their cart.

This is a huge customer irritant and I don't know of any other website that does this to their customers.
When an existing user has previously saved items in their shopping cart under their registered profile, then if they return to the website again WITHOUT registering and add items to the cart and then Sign-in, the items from the temporary cart should be ADDED to the items saved in the existing user's shopping cart under their registered profile and not overwrite the old cart contents.

I was told by Template Monster that this is related to the core Prestashop engine functionality and server cookies management.
How do we fix this?