Thanks for providing schema and step by step instructions to reproduce the issue.
I could reproduce this on master. I am accepting this as a bug. This in a way is similar to what @spinelsun has reported over here, Can't insert data .
Comment about your schema:
You could also use @secret directive for password field. You may read more about it here, https://dgraph.io/docs/master/graphql/schema/types/#password-type