Magento 2 – Salable Quantity Showing Zero in Product Grid

This is by design behavior as on the Category and Search Result pages Magento does not apply Inventory Reservations which are created when Order is placed. This is done to get the maximum performance on the category page. The product will disappear from the Category page as soon as you complete the order and create a shipment, that would lead to Source deduction and re-calculation of Stock Quantity

Based on your description the root cause of the problem could be one of those:

1. You have upgraded to Magento 2.3 from the previous version of Magento where you had some orders placed, but not shipped. As since Magento 2.3 the stock deduction logic has changed drastically and now Magento places reservation at the time of Order placement (negative reservation) and then compensate it at the time of Shipment creation (positive reservation), you face the problem that Magento compensates the reservation which in fact was not created (because an order has been placed on older version of Magento.). You can read more about this here - https://github.com/magento-engcom/msi/wiki/Salable-Quantity-Calculation-and-Mechanism-of-Reservations

2. another reason for getting this issue is that you had Inventory Configuration Manage Stock = false. With this config option, Magento does not place a reservation at the time of order placement. But then you decided to switch Stock management On, and placing shipment you overcompensated with positive reservation (similar as case 1)