If both BLE and Bluetooth are required to operate in the code, the initialization sequence seems to be critical.

Complete initialization of a BLE server and then separate initialization of Bluetooth Serial didn’t work. The correct sequence seems to be

Init BLE device => BLEDevice::init(apName);
Init Bluetooth Serial => SerialBT.begin(apName);
Then do the rest of the BLE initialisation like
- create server
- add callbacks to server
- add characteristics
- start server
- start advertising

Here is a code extract that worked for me

void initBlueTooth() {
  // Initialize BLE
  BLEDevice::init(apName);
  BLEDevice::setPower(ESP_PWR_LVL_P7);

  SerialBT.begin(apName);

  // Create BLE Server
  pServer = BLEDevice::createServer();

// ...
// initialize BLE callbacks and characteristics
// ...

  // Start the service
  pService->start();

  // Start advertising
  pAdvertising = pServer->getAdvertising();

  pAdvertising->start();
}

Full code example for the initialization can be found in this Github repo.