public class C
{
    public ObjectId Id;
    [BsonRepresentation(BsonType.ObjectId)] // applies to items not to array
    public string[] X;
}

var c = new C { X = new string[] { ObjectId.GenerateNewId().ToString(), ObjectId.GenerateNewId().ToString() } };
var json = c.ToJson();
Console.WriteLine(json);

var d = BsonSerializer.Deserialize<C>(json); // throws exception because of this bug
Console.WriteLine(d.ToJson());