Why my zero-stock product doesn't get hidden?
Hide-out-of-stock automation is trigger based, and it would not run by itself unless the conditions are met.
Therefore, in this case, only the product is ordered and its inventory level becomes to 0, then the automation will hide it. If the inventory is already 0 before, then it would not trigger the automation.
All the products that already have 0 stock level but are still published would not be handled by this automation. However, all the products that have 0 stock after new purchases will be hidden by the automation.