Posting here in case anyone has similar issue, took me a while to suss it.
You can use Dropbox to host some types of file for you; as long as you remember to change the dl=0 to a dl=1 on the end of the “share dropbox link” URL, most filetypes download straight away. Handy for the odd cheeky embedded MP4 file. (yep, you can stream MP4s from Dropbox into WebAR if you like. Handy, huh - and they don’t mind you doing it as long as you don’t flatten their server with traffic).
For some reason, though, if you try accessing a JSON file in this way, Dropbox insists on wrapping it in a bare bones HTML page.
So my tip of the day is: if you want to host JSON files on Dropbox, rename them so they don’t end in .json. Stick a few characters on the end of the extension - like mydata.json_h - and grab a new dropbox link.