1. Go to the product description page of a pre-order book (I did "Through Gates of Splendour")
2. Click on either "Quick Buy" or "Pre-order" (these both do the same thing, which is another bug I've reported earlier, so it doesn't matter which you choose; both will reproduce this)
3. Do not click "Place order" but instead click your browser's Back button.
4. Now click on "Cart"
Result: The book you added to your cart is no longer there, and you didn't explicitly delete it from your cart.
If I'm wanting to do some further shopping after one pre-order, my instinct would be to click Back in my browser and go find another book, place that one in my cart, etc., until I'm ready to check out. This works fine with all other books in Vyrso, but not pre-orders.