Why are my items switching to "Out of Stock" after order placed, even though 1000+ still in stock.
There are 2 source for quantity in magento : Quantity and Product Salable Quantity
Quantity : The quantity of the product decreases only after shipping is completed.
Product Salable Quantity : The salable quantity is updated when the physical quantity in warehouse changes, that is when new order is placed by the customer.
So, Please check once your Product Salable Quantity Section in your catalog product edit page. Also in case of multiple store check the quantity under particular stores scope.