using MongoDB.Driver;
using MongoDB.Driver.Linq;

var client = new MongoClient(new MongoClientSettings(){LinqProvider = LinqProvider.V3});
var collection = client.GetDatabase("some").GetCollection<Data>(nameof(Data));

collection.Aggregate().Group(data => new { data.Id, data.ProblemdData.Value }, d => d.Select(c => c)).ToList();

public class Data
{ public int Id { get; set; }
    public LikeNullable ProblemdData { get; set; }
}
public class LikeNullable
{
    public string Value { get; set; }
}