Thank you for getting in touch. You should be able to view the .json for POS orders in the same way you can view online orders. If you head to the Orders tab in your admin, and click on any order (including POS), you'll then just need to add ".json" to the end of the URL. It should read like this:
I just tried this on my test order with a POS order and the JSON data displayed correctly:
I will note that I have the JSONView Chrome extension installed so that the data here is more easily readable.
Hopefully you are able to do the same when checking this on your store, but if you have more questions on this please let me know.
Victor | Shopify Social Care