I have found a critical bug during the merge cart process on the demo site (BLC 2.2.1-SNAPSHOT and BLC 3.0.0, i have not tested on other version)
1/ Register a new customer
2/ Add a product in cart
3/ logout
4/ add a new product in cart
5/ login with the customer created previously
6/ THE PRODUCT CONTAINED IN THE CART IS NOT THE LAST PRODUCT ADDED IN CART
Other example :
1/ Register a new customer
2/ Add a product in cart
3/ Remove the product from cart
3/ logout
4/ add a new product in cart
5/ login with the customer created previously
6/ THE CART IS EMPTY when it should contain the last product added in cart
However, the order contained into the mergeCartResponse is correct, what is weird...
7/ After checkout the current cart, the cart contains the previous cart merged

Please let me know if you need more infos.