Order qty sometimes going through as 2 instead of 1

I have a script which submits 5 orders to a staging instance of Magento using MSI for the stock.

All 5 orders in the script are for the same product and with a Qty of 1 item. I noticed that the saleable Qty for an item was incorrect, but when I checked the inventory_reservations table, it became clear that the reason was due to some of the orders having a Qty of 2 instead of 1 as they should do, as the script only tells it to order 1 item.

I can also see when I check the detail for the orders in the admin, that some of the orders have somehow, come through with a Qty of 2.

I have absolutely no clues as to why this happening though. I have even tested with the module in app/code disabled. There are also (apart from standard magento module) no 3rd party module related to handling orders/stock.

Can someone help please?