Hi,
You'll find the same option when you edit your products listing menu/module via the menu Display>Content menus/modules and it overrides the setting of the global configuration (like any other setting of the menus/modules).
So I suppose that you must have that option turned off in the menu/module you're looking at.
There is no option to have it display during a certain period after it goes out of stock. It would require a custom HikaShop plugin to be developed in order to handle that.
