Swift pagination issue - swift

Question

I'm working on a chat app and the issue I'm having is with pagination. I'm able to load the first batch of documents and when I scroll to the top to load the next batch, it loads but instead of keeping the first batch of documents, it removes them. I'm not sure what I'm missing or doing wrong. Any help is greatly appreciated. Here is my code:
VC
var messages = [Message]()
var lastDocument: DocumentSnapshot? = nil
var fetchMoreDocs = false
func fetchChatMessages() {
fetchMoreDocs = true
var query: Query!
if messages.isEmpty {
query = MESSAGES_COLLECTION.document(currentUID!).collection(user.memberId).order(by: TIMESTAMP).limit(to: 5)
} else {
query = MESSAGES_COLLECTION.document(currentUID!).collection(user.memberId).order(by: TIMESTAMP).start(afterDocument: self.lastDocument!).limit(to: 5)
}
query.addSnapshotListener { (snapshot, error) in
if let error = error {
print("Error..\(error.localizedDescription)")
} else if snapshot!.isEmpty {
self.fetchMoreDocs = false
return
} else {
self.messages = Message.parseData(snapshot: snapshot)
DispatchQueue.main.async {
self.chatRoomTV.reloadData()
self.scrollToBottom()
self.fetchMoreDocs = false
}
}
self.lastDocument = snapshot!.documents.last
}
}
func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
let position = scrollView.contentOffset.y
let maxOffset = scrollView.contentSize.height - scrollView.frame.size.height
if maxOffset - position <= -50 {
fetchChatMessages()
}}
func scrollToBottom() {
if messages.count > 0 {
let index = IndexPath(row: messages.count - 1, section: 0)
chatRoomTV.scrollToRow(at: index, at: UITableView.ScrollPosition.bottom, animated: false)
}}