I generated my documentation with both options: from a live endpoint and from a json file. The behaviour is the same in both cases. Here is a screenshot of a mutation:

here is the same documentation for that mutation in the default graphiQl interface:

I'm also attaching the json generated file . On line 4259 seems to be where the args for the register mutation start going all the way to line 4368, so it feels the data is there, but somehow doc does not display it
api.txt
(Can't really upload json files here, so I converted to .txt)