It seems when an instance is loaded from a database, its `attributes`
method returns an instance of `BSON::Document`. This causes some
surprising behavior when we use `#add_to_set` on that loaded instance.

When a value is assigned to a `BSON::Document` instance, it gets
modified:

https://github.com/mongodb/bson-ruby/blob/master/lib/bson/document.rb#L87-L89

During modification, the attribute loses its reference to the original
value, however the `add_to_set` method relies on this reference:

https://github.com/mongodb/mongoid/blob/136ccbc140b25719f48ff5185bb471f90c063148/lib/mongoid/persistable/pushable.rb#L27-L31

Since that reference is broken, it no longer update the instance's
attribute and the value is lost in memory, though still retained in the
DB.

I made a PR here that I hope can help:

https://github.com/mongodb/mongoid/pull/4749