Bluetooth disconnecting after 1 second

OK, some progress on the original Bangle.js. It very rarely works (maybe 1 in 30 times), but I have observed it working and successfully reading out data from the characteristic:

NRF.connect("34:85:18:00:38:8e").then(function(gatt) {
  console.log(gatt);
  console.log("connected");
  var t = getTime();
  gatt.device.on('gattserverdisconnected', function(reason) {
    console.log("disconnected (" + reason + ") after " + (getTime()-t) + " secs");
  });
  return gatt.getPrimaryService("4fafc201-1fb5-459e-8fcc-c5c9c331914b");
}).then(function(service) {
  console.log("got service: ");
  console.log(service);
  return service.getCharacteristic("beb5483e-36e1-4688-b7f5-ea07361b26a8");
}).then(function(characteristic) {
  console.log("got characteristic, value = ");
  characteristic.readValue().then(function(v) { console.log(v); });
  characteristic.on('characteristicvaluechanged', function() {
    console.log("characteristicvaluechanged: ");
  characteristic.readValue().then(function(v) { console.log(v); });
  });
}).catch(function(e) {
  console.log("error: " + e);
});

gives:

BluetoothRemoteGATTServer: {
  "device": BluetoothDevice: {
    "id": "34:85:18:00:38:8e",
    "gatt":  ...
   },
  "connected": true }
connected
got service:
BluetoothRemoteGATTService: {
  "uuid": "4fafc201-1fb5-459e-8fcc-c5c9c331914b",
  "isPrimary": true, "start_handle": 40, "end_handle": 65535 }
got characteristic, value =
DataView: {
  "buffer": new Uint8Array([4, 0, 0, 0]).buffer,
  "byteOffset": 0, "byteLength": 4 }

However you'll notice that the "characteristicvaluechanged" callback is not firing. I left it running for several minutes and it didn't fire even once, so I think the "notify" response is not working. But this is progress, at least, and in the worst case I can probably read out the characteristic value on a timer, so there is light at the end of the tunnel!

It would still be better if it worked on Bangle.js 2, and if it worked every time instead of only 1/30 times.

(The MAC address is different because I'm using a different ESP32C3, but that's just an accident - it still doesn't work on the Bangle.js 2).