I wonder if this is a bug or a will ?
When a customer has products in its wishlist on his account : if he add another product in a wishlist without beging connected and the connect to its account, the registred wishlist of the account is deleted and replaced by the product placed in wishlist when not connected.
A customer has just lost all the products he had in his wishlist with that manipulation 
I have not been able to reproduce the issue described. What version are you using? If older than 4.16.1, please try to upgrade to at least that version.
I am using CS-Cart 4.18.2 : test made with Firefox 131.0.2
Here’s what I’ve done :
Log on my account.
Add some products in wishlist.
Log out
Add another product in wishlist
Log in my account
The wishlist has been replaced with the new product (and old ones are lost)
Let me know if you want I make a video if you cannot reproduce it.
Can not reproduce this issue. After the mentioned steps, guest wishllist is combined with the logged in user. You may need to check if it’s a conflict with other 3rd party addon or custom mods.
I tested on another website (4.10) and it works fine (unlogged wishlist is added to the connected one when we log).
You’re perhaps right; it could come from a 3rd party add-on.
Will investigate in that way.
Thank you for checking.