This matter seems it has yet to been resolved.
https://community.shopify.com/c/shopify-apis-and-sdks/why-inventory-quantity-is-still-writable-in-product-api-put/m-p/533974
If you don’t set that field, “inventory_quantity”, the quantity is initialized by 0 that means default value. I also tried to set Null.
This must be a bug only for API because if you upload any product via CSV file and set blank that field(Variant Inventory Qty), the quantity remains.
I’d like to keep the quantity as well in case of API. I need your further investigation.