Can you post the rails code for your controller? It seems like for some reason it is skipping the bootstrap part. Normally if a request comes in that doesn’t include .json (or an accept header of JSON like from AJAX) Discourse will bootstrap the ember app by serving HTML. If you are getting JSON without a .json that tells me you have somehow disabled this. Maybe by skipping a filter?
I noticed you are trying to append to the user route without using a resource parameter. Discourse-assign does it like this:
I would try that approach.