Flutter Firebase Cloud Functions deleting all items as soon as they are written - firebase

Question

I'm trying to use the deleteOldItems Cloud Functions code for my project, when I use the code and simply change the 2 hours to 4 it deletes all my nodes as soon as they are written. Here is my Firebase functions code...
exports.deleteOldItems = functions.database.ref('messages/{pushId}').onWrite(async (change) => {
var ref = change.after.ref.parent; // reference to the items
var now = Date.now();
var cutoff = now - 4 * 60 * 60 * 1000;
var oldItemsQuery = ref.orderByChild('timestamp').endAt(cutoff);
return oldItemsQuery.once('value', function(snapshot) {
// create a map with all children that need to be removed
var updates = {};
snapshot.forEach(function(child) {
updates[child.key] = null
});
// execute all updates in one go and return the result to end the function
return ref.update(updates);
});
});
And here is how I'm saving my timestamp in Flutter to
database/messages
with child 'timestamp'..
var time = DateTime.now().millisecondsSinceEpoch;
Here is my JSON # 'messages'
"messages" : {
"-M3e5Glv5JEaz12hDwBB" : {
"body" : "Hello",
"name" : "Charles",
"photo" : "https://media1.giphy.com/media/7wDEaTbCupuRq/giphy-downsized.gif",
"timeStamp" : 1585549220985,
"user" : "4eb8920a-e407-4488-bce4-c6f64f7b0891"
},
"-M3e5Jfx2cnbtMTNLPev" : {
"body" : "Example",
"name" : "Charles",
"photo" : "https://media1.giphy.com/media/7wDEaTbCupuRq/giphy-downsized.gif",
"timeStamp" : 1585549232893,
"user" : "4eb8920a-e407-4488-bce4-c6f64f7b0891"
},
"-M3e6oYk6_zTe6IAs0Fg" : {
"bodyGif" : "https://media3.giphy.com/media/8xomIW1DRelmo/giphy.gif",
"name" : "Charles",
"photo" : "https://media1.giphy.com/media/7wDEaTbCupuRq/giphy-downsized.gif",
"timestamp" : 1585549625584,
"user" : "4eb8920a-e407-4488-bce4-c6f64f7b0891"
},
"-M3hR_DrGcKhM8zpFbYA" : {
"body" : "Again",
"name" : "Charles",
"photo" : "https://media1.giphy.com/media/7wDEaTbCupuRq/giphy-downsized.gif",
"timeStamp" : 1585605399479,
"user" : "4eb8920a-e407-4488-bce4-c6f64f7b0891"
}
and my database security rules are as follows for 'messages'...
"messages": {
".indexOn": ["timestamp"]
}
How can I get the nodes to be removed as expected?