Did you find any log while placing order ?
may be related to inventory?
Which version are you using?
Did you installed any third party plugin or any customization related to inventory?
You should debug with your local environment.
try to identify different between both products.
It happens in some circumstances where order loses orderitems. SOmetimes happens when impersonating a customer, and in this case you find orderitems in your account.
When orderitems are lost, they are more likely to be assigned to another order.
The solution I use when I suspect this behaviour is to investigate into the database: summing unitprice multiplied by quantity should mach the value of the cart for each order. Be sure to choose the right fields, you must test in subset known to check everything.