C# MongoDb get document by id not working

ConfigMongoDb.cs

using MongoDB.Bson.Serialization.Conventions;

namespace Judasheet
{
    public static class ConfigMongoDb
    {
        public static void SetupConvention()
        {
            var pack = new ConventionPack();
            pack.Add(new CamelCaseElementNameConvention());
            ConventionRegistry.Register(
                "CamelCase",
                pack, type => type.FullName.StartsWith("Judasheet.Models"));
        }
    }
}

DbTests.cs

using System;
using System.Linq;
using Judasheet;
using Judasheet.Models;
using MongoDB.Bson;
using MongoDB.Driver;
using Xunit;

namespace JudasheetTest
{
    public class DbTest
    {
        private const string DatabaseName = "test";

        private IMongoCollection<T>GetSongsCollection<T>(string collectionName = null)
        {
            ConfigMongoDb.SetupConvention();
            var client = new MongoClient(Environment.GetEnvironmentVariable("TEST_JUDA_MONGODB_URI"));
            var database = client.GetDatabase(DatabaseName);
            var name = collectionName ?? typeof(T).Name.ToLower();
            return database.GetCollection<T>(name);
        }
            
        [Fact]
        public void ListAll_ResultNotEmpty()
        {
            var songs = GetSongsCollection<Song>().FindSync(_ => true).ToList();
            Assert.NotEmpty(songs);
        }

        [Fact]
        public void GetById_Workaround_NotNull()
        {
            var existingSong = GetSongsCollection<Song>().FindSync(_ => true).First();
            var song = GetSongsCollection<Song>()
                .AsQueryable()
                .Single(s => s.Id.StartsWith(existingSong.Id));
        }
        
        // does not work!
        [Fact]
        public void GetById_NotNull()
        {
            var existingSong = GetSongsCollection<Song>().FindSync(_ => true).First();
            
            var song = GetSongsCollection<Song>()
                .Find(s => s.Id == existingSong.Id)
                .Single();
        }

        [Fact]
        public void GetByIdBson_NotNull()
        {
            var songs = GetSongsCollection<Song>();
            var existingSong = songs.FindSync(_ => true).First();
            var filters = Builders<BsonDocument>.Filter.Eq("_id", existingSong.Id);
            
            GetSongsCollection<BsonDocument>("song").Find(filters).First();
        }
    }
}

Song.cs

using System;
using System.Collections.Generic;
using MongoDB.Bson;
using MongoDB.Bson.Serialization.Attributes;

namespace Judasheet.Models
{
    public class Song
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public string Id { get; set; }

        public DateTime InsertDate { get; set; }

        public DateTime UpdateDate { get; set; } = DateTime.Now;

        public string Title { get; set; }

        public string MainLanguage { get; set; }

        public string SongType { get; set; }

        public string SongWriter { get; set; }

        public string Album { get; set; }

        public List<Mastersheet> Mastersheets { get; set; }

        public bool Verified { get; set; } = false;
    }
}