using MongoDB.Bson.Serialization.Conventions;
using MongoDB.Driver;
using MongoDB.Driver.Linq;
var defaultPack = new ConventionPack { new CamelCaseElementNameConvention() };
ConventionRegistry.Register("camelCase", defaultPack, c => true);
var client = new MongoClient("mongodb://localhost:27017");
var collection = client.GetDatabase("kvp_bug").GetCollection<Person>(nameof(Person));
await collection.InsertOneAsync(new Person() { Title = "Other" });
await collection
    .AsQueryable()
    .GroupBy(x => x.Title)
    .Select(b => new KeyValuePair<string, int>(b.Key, b.Count()))
    .ToListAsync();
public class Person
{
    public string Title { get; set; }
}