How MongoClient::save(...) might change the _id field of document parameter - vert.x

Question

I have a class User that embeds a JsonObject to represent the user's fields. This class looks like that:
class User {
private JsonObject data;
public User(...) {
data = new JsonObject();
data.put("...", ...).put(..., ...);
}
public String getID() { return data.getString("_id"); }
// more getters, setters
// DB access methods
public static userSave(MongoClient mc, User user){
// some house keeping
mc.save("users", user.jsonObject(), ar -> {
if(ar.succeeded()) { ... } else { ... }
});
}
}
I've just spent more than half a day trying to figure out why a call to user.getID() sometimes produced the following error: ClassCastException: class io.vertx.core.json.JsonObject cannot be cast to class java.lang.CharSequence. I narrowed down to the userSave() method and more specifically to MongoClient::save() which actually produces a side effect which transforms the data._id from something like
"_id" : "5ceb8ebb9790855fad9be2fc"
into something like
"_id" : {
"$oid" : "5ceb8ebb9790855fad9be2fc"
}
This is confirmed by the vertx documentation which states that "This operation might change _id field of document parameter". This actually is also true for other write methods like inserts.
I came with two solutions and few questions about doing the save() properly while keeping the _id field up to date.
S1 One way to achieve that is to save a copy of the Json Object rather than the object itself, in other words : mc.save("users", user.jsonObject().copy(), ar -> {...});. This might be expensive on the long run.
S2 An other way is to "remember" _id and then to reinsert it into the data object in the if(ar.succeeded()) {data.put("_id", oidValue); ...} section. But as we are asynchronous, I don't think that the interval between save() and the data.put(...) is atomic ?
Q1: Solution S1 make the assumption that the ID doesn't change, i.e., the string 5ceb8ebb9790855fad9be2fc will not change. Do we have a warranty about this ?
Q2: What is the right way to implement the saveUser() properly ?
EDIT: The configuration JSON object user for the creation of the MongoClient is as follows (in case there is something wrong) :
"main_pool" : {
"pool_name" : "mongodb",
"host" : "localhost",
"port" : 27017,
"db_name" : "appdb",
"username" : "xxxxxxxxx",
"password" : "xxxxxxxxx",
"authSource" : "admin",
"maxPoolSize" : 5,
"minPoolSize" : 1,
"useObjectId" : true,
"connectTimeoutMS" : 5000,
"socketTimeoutMS" : 5000,
"serverSelectionTimeoutMS" : 5000
}