Leveraging Shopify Predictive Search API

I’m trying to implement the predictive search API to create a custom solution when a product is out of stock. The idea is that we want to show an alternate product OF OUR CHOOSING (i.e. not using the automated recommendations). We have added a tag to all products with an alternate SKU that we want to show if an item is OOS. The Predictive Search API seems the perfect solution for this, but I can’t get it to give me any results.

I found this forum post that seems to answer this exactly, but it doesn’t work for me.

Here is the URL structure I’m trying to use:

https://example.com/search/suggest.json?q=searchstring&resources[type]=product&options[fields]=title

When I send this, either from Postman or from a page on the site, I get this:

{
    "resources": {
        "results": {
            "products": []
        }
    }
}

HOWEVER…

If I alter the URL slightly to request the predictive search HTML snippet (as is done in the theme), it returns results:

https://example.com/search?view=ajax&q=searchstring&options[prefix]=none&type=product

Does anyone know what’s wrong here? Why is the HTML version of the API working but the JSON version isn’t? Am I doing something wrong? Is there a setting somewhere I missed?