Since version v1.0.0-alpha40 it is impossible to use relations inside mutation returning.
Example:

mutation {
  insert_smth_v1(objects: [{name: "foo", relate_id: 1}]) {
    returning {
      relateByrelateId {
        id
      }
    }
  }
}

Will return "message": "field \"relateByrelateId\" not found in type: 'smth_v1_columns'"
But it is still posible to use it inside of the query:

{
  smth_v1{
    relateByrelateId {
        id
      }
  }
}