Strange behaviour when self referencing objects in grails and mongodb

I have the following self referencing domain class:

package com.test

import org.bson.types.ObjectId

class Category {
ObjectId id
String name
Category parent = null
static hasMany = [childs: Category]
static mappedBy = [ childs: 'childs' ]
}

Then, I create 2 "Category" objects, where the 2nd object becomes child of the first, like this:

def c1 = Category.findByName('category1') ?: new Category(name: 'category1').save(failOnError: true)
def c2 = Category.findByName('category2') ?: new Category(name: 'category2', parent: c1).save(failOnError: true)
c1.addToChilds(c2).save(failOnError: true)

The outcome, is not what I expect and it appears that both objects become the child of the another:

{
"_id" : ObjectId("5b8d0ba306425e3ee4ff5cf9"),
"name" : "category1",
"version" : NumberLong(0),
"childs" : [
ObjectId("5b8d0ba306425e3ee4ff5cfa")
]
}

{
"_id" : ObjectId("5b8d0ba306425e3ee4ff5cfa"),
"name" : "category2",
"version" : NumberLong(0),
"childs" : [
ObjectId("5b8d0ba306425e3ee4ff5cf9")
],
"parent" : ObjectId("5b8d0ba306425e3ee4ff5cf9")
}

And while I can simply ignore children that are also parents, why I get this odd behavior?

Start with the Category self-reference, its hasMany and mappedBy declarations, and the shown sequence that creates, links, and saves c1 and c2. Reproduce it against MongoDB and compare the persisted parent and childs references; done means explaining why both documents contain the reciprocal reference or identifying the mapping behavior responsible.

Written by the indexing model from the issue text.