public abstract class Foo
{
    [BsonId]
    public ObjectId Id { get; set; }

    [BsonElement("type")]
    public abstract string Type { get; }
}

public class Bar : Foo
{
    public override string Type => "bar";
}

public class FooDiscriminatorConvention : IDiscriminatorConvention
{
      public string ElementName => "type";
      public Type GetActualType(IBsonReader bsonReader, Type nominalType) => typeof(Bar);
      public BsonValue GetDiscriminator(Type nominalType, Type actualType) => "bar";
}

BsonSerializer.RegisterDiscriminatorConvention(typeof(Foo), new FooDiscriminatorConvention());

var client = new MongoClient("mongodb://localhost:27017");
var collection = client.GetDatabase("test").GetCollection<Foo>("foos");  

// 1st request - THROWS BsonSerializationException (correct behavior)
try
{
    await collection.Find(FilterDefinition<Foo>.Empty).ToListAsync();
}
catch (BsonSerializationException ex)
{
    Console.WriteLine($"1st request threw: {ex.Message}");
}

// 2nd request - SUCCEEDS silently (inconsistent behavior!)
var results = await collection.Find(FilterDefinition<Foo>.Empty).ToListAsync();
Console.WriteLine($"2nd request succeeded with {results.Count} items");