fresh start with MDBT42Q , some specs before start toe code

but on the OLED I still see "Connected" and the temp is changing every 5 seconds. I guess it's because the Interval function that is still running

Yes... so in BLE_Connect you call setInterval but you never clear that interval.

Try:

var updateInterval;

function BLE_Connect(){ 
  updateInterval = setInterval(function() {
  ....
}

function BLE_Disconnect(){
  if (updateInterval) clearInterval(updateInterval);
  updateInterval=undefined;
  ...
}