I'd like to report a problem I am facing.
When update product title and variants inventory quantities at the same time through the Shopify Admin product details page. The product/update webhook triggered does not include changes for all variants, even though the inventory quantity for those variants were changed.
I managed to replicate the problem every time with the followng steps:
I am not Shopify but to add to your simple scenario, you might want to explore a bit more?
You have to be sure your target is not moving. Inventory is fluid. There is inventory sitting there, on the shelf, and then there is inventory sitting in carts, off the shelf. You don't see those unless you check carts too. So by saying you changed a 3 to a 4, but the webhook threw a 3, does not mean much if one is also in a cart.
Maybe this is not your case.. but I am throwing that out there... playing with inventory and webhooks is NOT simple. It involves many moving parts, none of which are reliable unless you have aced all the inner workings of delta changes vs. static changes, with cart quantities too. And now we also have that new endpoint for Inventory too where future deliveries are involved. Phew...
Yes I do understand that there are many moving parts when it comes to inventory quantiry, but I tested the scenario on my developement store, there is no other traffic other than from my test. I am sure there isn't other factor (in cart, etc) that could influence the inventories in my test case.
It only happend if I change product title and multiple variants quantities at the same time. Changing only variants quantities there is no issue.
The only way to make sure correct inventory quantities (assuming no other factors involved) is when a webhook (product/update) is triggered, app will do another API call to fetch the product in question. Which is not good.