This drove me crazy, but I think I have figured out the fix to get this to work. I noticed that an old virtual switch I created a while ago would show up in Stringify, but newly created virtual switches would not. I banged my head on my keyboard for an hour comparing them to try to find the difference. Then I noticed in the SmartThings graph.api that the one showing up in Stringify had a current state of "switch: off" while the new one had no state listed at all. So I found the thing in my SmartThings mobile app, and where it would normally show On or Off, it showed --- instead. I toggled the new virtual switch off/on. Now the device shows a current state in the SmartThings API, and it can be added to Stringify.