public class ReplaceWithBug
{
    [Fact]
    public void ReplaceWithShouldRenderElementNameCorrectly()
    {
        BsonClassMap.RegisterClassMap<User>(cm =>
        {
            cm.AutoMap();
            cm.MapMember(c => c.UserId).SetElementName("uuid");
        });

        var client = new MongoClient(new MongoClientSettings { LinqProvider = MongoDB.Driver.Linq.LinqProvider.V3 });
        var collection = client.GetDatabase("db").GetCollection<User>("user");

        var rendered = PipelineStageDefinitionBuilder
            .ReplaceWith<User, User>(u => new User { UserId = u.UserId })
            .Render(collection.DocumentSerializer, collection.Settings.SerializerRegistry, collection.Database.Client.Settings.LinqProvider);

        var bson = rendered.Document.ToString();

        Assert.Equal("""{ "$replaceWith" : { "uuid" : "$uuid" } }""", bson);
    }
}

public class User
{
    public Guid UserId { get; set; }
}

  Message: 
    Assert.Equal() Failure: Strings differ
                                     ↓ (pos 22)
    Expected: "{ "$replaceWith" : { "uuid" : "$uuid" } }"
    Actual:   ···""$replaceWith" : { "UserId" : "$uuid" } }"
                                      ↑ (pos 22)