When connected to a server whose version is <= 3.0, a $explain key added to FindOptions.modifiers will be included in the OP_QUERY document. But with the introduction of the find command in server 3.2, a $explain modifier must be converted into an explain command instead, as the find command does not support explain.

So this code:

MongoCollection<Document> collection = ...
collection.find().modifiers(new Document("$explain", true)).first()

should return the query plan, but when connected to a 3.2 server, it does not. It will instead return the first document from a normal cursor on the collection.