Had a closer look, and there is an argument restock (boolean) for the orderEditSetQuantity mutation (docs here). This isn’t required, but that will need to included for input and set to TRUE if the intention is to restock the removed item(s).
Our developer and API examples like the one shared, offer an excellent starting point to explore specific functionalities on the platform, but suggest exploring the indepth API documentation for added insights. These include more detailed docs for objects, queries and mutations as well as available fields, arguments, expected values, and connections among other important details. Here is the main page for the Shopify GraphQL Admin API docs.