There would be one possible reason for this case. If you are using the function of Discount and Partial payment in the old preorder model, the app system will only assist to buy 1 pre-order product
When clicking to Pre-order button, you will be redirected to Check-out page and skip the cart page
For your reference: https://globo.io/kb/how-to-add-the-partial-payment-discount-products-to-the-cart-to-purchase-many-products-at-the-same-cart