Hi Owen Dwyer,
Thanks for sharing your experience. The "malformed JSON" error message is a bug that we are in the process of fixing.
You may have been encountering an error because you were trying to derive locations from multiple different layers at once. While you can use multiple layers to build your expressions, all expressions should relate back to only one layer from which you are trying to create a subset.
Let me know if you have any further questions!
Cheers,
Amanda
Product Engineer, Esri
Amanda
Product Engineer - Web Analysis