What problem are you facing?

On mongodb@3 we used the "readPreferenceTags" set using th options param of the MongoClient. It was an array: ex:

MongoClient( uri, { "readPreferenceTags" : [{"dc" : "irl.amz"}]})

After updating the driver to mongodv@4.2.1, tags are processed correctly. 

What driver and relevant dependency versions are you using?

mongodb@4.2.1

Steps to reproduce?

await (new MongoClient(        URI,        {            readPreference : "nearest",            readPreferenceTags : [{"dc":"irl.amz"}]        }    ).connect());

Put a breakpoint in connection_string.js at line 790

Observe that the readPreferenceTag array is populated with an empty object: {}. 

That happens because the transform function gets the tags as an Array of Array of objects, and not Array of objects, as it should.