We’re having issues with many products showing as in stock/shoppable when they’re actually out of stock. We have our location settings to only fulfill orders from our ‘Warehouse’ location, we have sell when out of stock switched off. When you proceed to checkout it does luckily flag that the items are out of stock and removes them from basket.
On our dev theme I added {{ current_variant.available }} & {{ current_variant.inventory_quantity }} to see what it says - products with 0 or negative stock are returning yes for available and an inventory quantity number that doesn’t seem to reflect any of our stock levels at other locations etc. I installed Horizon to check it’s not a theme issue, and the horizon theme also shows out of stock products as available and allows me to add the products to cart.