You'll need to add the created object to a cell in the sheet properties as well. I recommend you download this tool:
Through that tool you can navigate to the sheet object and look at it's properties. That should give you a clearer picture of how the sheet object stores its properties.
I'm not sure what technology you are using for your API calls, but if you use the .Net SDK you can also leverage this method which will make sure the created object is added correctly to the sheet properties as well: