Firebase cloud functions stop working if remain idle for 5 mins

Question

I have written this simple cloud function to update the data in one node everytime data in another node gets updated.

exports.trending = functions.database.ref("/trending/{tick}")
.onUpdate(event => {

     const ticker = event.params.tick;
     const path = "/symbols/exchange/" +ticker;
     console.log("Create trending list " + ticker);
     const quotes_list   = event.data.ref.root.child(path);

     return quotes_list.on("child_changed", function(snapshot){
            console.log("Passing " + event.data.ref);
            console.log("Passing " + snapshot.val());
            console.log("Passing " + ticker);
            updateTicker(snapshot, event.data.ref, ticker);
     });

});

function updateTicker(snapshot, ref, ticker){
  if(snapshot.toJSON() !== null){
    console.log("Rcvd " + ref);
    return ref.update({last_price: snapshot.val()}).then(noUse => { console.log("Event triggered " + snapshot.child("name").val()); });
  }else{
    console.log("data is not available for " + ticker);
    return;
  }
}

Problem is: 1. on("child_changed", snapshot) ==> only contains data that is changed. Not the complete snapshot. 2. My cloud function works fine for first 5-10 mins.But if it remains idle for ~5 mins, It stops working? Is there something wrong I am doing here? Please help.