I was able to create a product with the inventory level without a problem, when I followed your instruction. (using the endpoint for Product, not for Product Variant). However, I saw a warning notice regarding 'inventory_quantity' on the top of the Product Variant page. I am currently assuming that it is not possible to create a product variant with the inventory level, using the 'Product Variant' endpoint, at the moment, but you can create it when using the "Product" endpoint.
Am I correct on this? By the way, I am using Admin API (Restful Version) instead of GraphQL version.