3.1 got released a couple of hours ago and took down my server. The issue looks to be a bug in 3.1 that is triggered by a bug in my code.

For some reason my code was doing:

const collection = this.db.collection(name, name);

i.e. passing a string to the options arg. This worked in 3.0 but breaks in 3.1. The reason is that in 3.1 the shallow copy is broken.

3.0: https://github.com/mongodb/node-mongodb-native/blob/3.0/lib/db.js#L412
3.1: https://github.com/mongodb/node-mongodb-native/blob/master/lib/db.js#L316
 
Not the object.assign isn't assigned to options? I presume it should read
options = Object.assign({}, options);