Using the Node Driver, I create a connection with the "useUnifiedTopology" option set to true with a snippet like this.

  const { url } = databaseConfig;  
  const databaseConnection = new MongoClient(url, {
     useUnifiedTopology: true,
     useNewUrlParser: true,
   });
   return databaseConnection.connect();

Later I close the connection and check to see if it's closed:

await client.close();
expect(client.isConnected()).toBe(false); // this fails because it's true

When tracing the isConnected() call it ends up returning true (always) from here:

https://github.com/mongodb/node-mongodb-native/blob/dc70c2de7d3dae2617708c45a1ea695d131e15f3/lib/core/sdam/topology.js#L663

If I change the useUnifiedTopology to false and leave everything else the same then the code works as expected and this line is hit instead:

https://github.com/mongodb/node-mongodb-native/blob/dc70c2de7d3dae2617708c45a1ea695d131e15f3/lib/topologies/topology_base.js#L340