Why does my Recyclerview add new items at random positions?

After much reading on how to add an item to the top (or bottom), I realized that list.add(position, item) along with notifyDataSetChanged() is the correct way to add an item at the top (index 0). However, in my code, even after doing that, my RecyclerView adds positions at random places. Below is my CommentsActivity code:

class CommentsActivity : AppCompatActivity() {

private var clickedPostUid = ""
private var currentUserUid = ""
private lateinit var binding_commentsActivity: ActivityCommentsBinding
private val currentUserUID = FirebaseAuth.getInstance().currentUser?.uid!!
private val dbRef = FirebaseDatabase.getInstance().reference
private val uidRef = currentUserUID.let { dbRef.child("Users").child(it) }
private var commentAdapterView: CommentsAdapter? = null
private var commentsList: MutableList<CommentsModel>? = null
var commentsActivityRecyclerView: RecyclerView? = null

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    binding_commentsActivity = ActivityCommentsBinding.inflate(layoutInflater)
    clickedPostUid = intent.getStringExtra("keyIdentifierPostUid")!!
    currentUserUid = intent.getStringExtra("keyIdentifierCurrentUserUid")!!

    commentsActivityRecyclerView =
        binding_commentsActivity.commentsActivityRecyclerView


    val layoutManager = LinearLayoutManager(this)
    layoutManager.reverseLayout = true
    layoutManager.stackFromEnd = true

    commentsActivityRecyclerView!!.layoutManager = layoutManager

    commentsList = ArrayList()
    commentAdapterView = CommentsAdapter(this, commentsList as ArrayList<CommentsModel>)
    commentsActivityRecyclerView!!.adapter = commentAdapterView
    commentsActivityRecyclerView!!.setHasFixedSize(true)

    LoadAndDisplayComments()

And this is my LoadAndDisplayComments()

private fun LoadAndDisplayComments() {
    val commentsRef = dbRef.child("Comments").child(clickedPostUid)

    commentsRef.addValueEventListener(object : ValueEventListener {
        override fun onCancelled(p0: DatabaseError) {
            TODO("Not yet implemented")
        }

        override fun onDataChange(p0: DataSnapshot) {
            if (p0.exists())
            {
                commentsList!!.clear()
                for (snapshot in p0.children)
                {
                    val getCommentDetails = snapshot.getValue(CommentsModel::class.java)
                    if (getCommentDetails != null) {
                        commentsList!!.add(0,getCommentDetails)
                    }
                }

                commentAdapterView!!.notifyDataSetChanged()
            }
            else{
                Toast.makeText(this@CommentsActivity, "No comments" , Toast.LENGTH_SHORT).show()
            }
        }
    })
}

This is my comments adapter

class CommentsAdapter(
private var commentsContext : Context,
private var commentsList: MutableList<CommentsModel>
) : RecyclerView.Adapter<CommentsAdapter.CommentsViewHolder>()
{
inner class CommentsViewHolder(@NonNull itemView: View) : RecyclerView.ViewHolder(itemView) {
    val showComment : TextView = itemView.comments_recview_item_comment
    val showImage : CircleImageView = itemView.comments_recview_item_profile_image
    val showUsername : TextView = itemView.comments_recview_item_username

}

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): CommentsViewHolder {
    val itemView = LayoutInflater.from(commentsContext)
        .inflate(R.layout.comment_recyclerview_item, parent, false)

    return CommentsViewHolder(itemView)
}

override fun getItemCount(): Int {
    return commentsList!!.size
}

override fun onBindViewHolder(holder: CommentsViewHolder, position: Int) {

    val currentUser = FirebaseAuth.getInstance().currentUser!!
    val comment = commentsList!![position]
    holder.showComment.text = comment.comment
}

I need that a new comment to be displayed at the top or bottom of the recyclerview. I know how to scroll to 0 index using .SmoothScroll. Would be happy to post the XML if needed. Thank you.

Turns out the error was not in the RecyclerView. It was the way I was storing the values in the database. I was creating a unique Id for my values in Firebase that was causing the error. Instead I used .push to generate unique keys and it worked fine.

Wrong approach (or just not what I wanted):

val uniqueCommentId = UUID.randomUUID().toString()
        .replace("-","").toUpperCase(Locale.ROOT)
    val postCommentsRef = FirebaseDatabase.getInstance().reference
        .child("Comments")
        .child(postId).child(uniqueCommentId )
   postCommentRef.setValue()

Better approach:

 val postCommentsRef = FirebaseDatabase.getInstance().reference
        .child("Comments")
        .child(postId)

    val commentMap = HashMap<String, Any>()
    commentMap["userUid"] = currentUserUid
    commentMap["postId"] = postId
    commentMap["comment"] = comment

    postCommentsRef.push().setValue(commentMap)

Worked fine for me after this.