Why is my Shopify app not updating stock quantities correctly?

[julius4]

I'm developing an app that provides product list and stock quantities for a shop in Shopify.

Everything was well until some time in the past when stock quantity updates stopped working.

From my app's access logs, I see that Shopify still sends a request to my app for stock quantities. I see a request that was made to:

GET /fulfillment/fetch_stock.json?max_retries=3&shop={shop-id}&sku={product-sku}&timestamp=1638777368

And when I try it, that exact request returns response:

{
    "{product-sku}": 2
}

 But the stock quantity is still 0 at shopify store. (NOTE: There are no unfulfilled orders in shopify that could decrease the stock quantity).

Any ideas on how to debug this, how to find out what is wrong? Maybe shopify's request is being blocked by my hosting provider, or maybe they send some kind of unusual headers that makes the request invalid and it fails? As I'm running out of options. It used to work well for quite some time, but suddenly stopped, even though no changes has been made in the app.