Hi smoothrider
Actually, this is how the system work if you have a value on HOLD STOCK field (Woocommerce settings > Products > Inventory).
The hold stock is a woocommerce feature that reserves the stock temporally (the minutes you put in the filed). In this way, if a user goes to the checkout without paying, it avoids that if another user wants to buy the product before, the first user can have his/her product available.
So, if you go to Paypal, the product is on hold until you pay, and if you go back, it's correct that there is an item less.
It can happen that Woocommerce, in some ways checked that it is about the same user so it doesn't take it into account, in any case, is a Woocommerce feature and ATUM has nothing to do with this.
Best Regards,
Andrea.