https://mongodb.github.io/node-mongodb-native/3.3/api/ClientSession.html#withTransaction says: 
IMPORTANT: This method requires the user to return a Promise, all lambdas that do not
return a Promise will result in undefined behavior.

The first Node.js example on https://docs.mongodb.com/manual/core/transactions/index.html says:

// Note: The callback for withTransaction MUST be async and/or return a Promise.

The example uses an async function and does not return a Promise.  I've tried this out and it's working for me. 

I think the driver docs need to be updated to indicate the function can be async or return a promise.