From driver documentation for find_and_modify operation at http://mongoc.org/libmongoc/current/mongoc_collection_find_and_modify.html?highlight=find_and_modify:

"Returns either the document before or after modification based on the _new parameter."

However, as far as I checked what is returned (in the reply parameter) is not the document, but a kind of "wraper" with the document:

{
	"lastErrorObject": {
		"n": 1,
		"updatedExisting": true
	},
	"value": <here is the document>,
	"ok": 1.0
}

Not this is not the behaviour if we use findAndModify() in the MongoShell. Example:

> db.x.insert({x:1})
WriteResult({ "nInserted" : 1 })
> db.x.findAndModify({query: {x:1}, update: {$inc: {x:1}}, new: true })
{ "_id" : ObjectId("615acd3a3b41c7be44a3374e"), "x" : 2 }

I understand the driver documentation should explain this (thus, this issue is a kind of "documentation bug").