is_ajax_request() doesn't work from mobile apps

I'm not sure if this is expected or if there's an alternative, but is_ajax_request() doesn't work from when handling an ajax request from a mobile app.

I'm developing an app using Phonegap, with my existing CI3 site as the back end. I have several methods setup to return data as JSON (these are also used in the normal web-based version of the app). I prevent direct access to these using an is_ajax_request() check, but this is failing when called from my mobile app (returns false).

Anyone know if there is a workaround or fix for this?

Thanks