I updated my project MongoDb C# driver to version 3.5.0 (from 3.4.1) and I started receiving this exception:

MongoDB.Bson.BsonSerializationException: 'The discriminator element name cannot be SecurityType because it is already being used by the property SecurityType of type InfrastructureInterfaces.Models.FutureIdentifierSheet'

FutureIdentifierSheet inherit from IdentifierSheet

public record IdentifierSheet
{
    public required string Identifier { get; init; }
    public Security SecurityType { get; init; }
    public required string Symbol { get; init; }
    public Exchange Exchange { get; init; }
    public string? Name { get; init; }
}

public record FutureIdentifierSheet : IdentifierSheet
{
    public string Underlying { get; init; } = string.Empty;

    public required DateTime ExpirationDate { get; init; }

    public required string ProductCode { get; init; }

    public required DateTime YearMonth { get; init; }
}

and this is my custom discriminator

private class SecurityTypeDiscriminatorConvention : IDiscriminatorConvention
{
    public string ElementName => nameof(IdentifierSheet.SecurityType);

    public Type GetActualType(IBsonReader bsonReader, Type nominalType)
    {
        var bookmark = bsonReader.GetBookmark();

        bsonReader.ReadStartDocument();
        var actualType = typeof(IdentifierSheet);

        while (bsonReader.ReadBsonType() != BsonType.EndOfDocument)
        {
            string? name = bsonReader.ReadName();
            if (name == ElementName)
            {
                var enumValue = (SecurityType.Security)(object)bsonReader.ReadInt32();
                actualType = enumValue switch
                {
                    SecurityType.Security.Future => typeof(FutureIdentifierSheet),
                    SecurityType.Security.Option => typeof(OptionIdentifierSheet),
                    _ => actualType
                };
                break;
            }

            bsonReader.SkipValue();
        }

        bsonReader.ReturnToBookmark(bookmark);
        return actualType;
    }

    public BsonValue GetDiscriminator(Type nominalType, Type actualType)
    {
        if (actualType == typeof(FutureIdentifierSheet))
            return (int)SecurityType.Security.Future;
        if (actualType == typeof(OptionIdentifierSheet))
            return (int)SecurityType.Security.Option;

        return (int)SecurityType.Security.NoSecurity; // fallback or base
    }
}

the execution code after class map and discriminator registration is

 var queryable = _futureSheetsCollection.AsQueryable();
 var query = queryable.Where(f => identifiers.Contains(f.Identifier));
 var sheets = await query.ToListAsync();

with an example document

{
  "_id": {
    "$oid": "68d50e6bd43281b1daa42b15"
  },
  "Identifier": "BBG002CYRBV9",
  "SecurityType": 6,
  "Symbol": "ES_H1998",
  "ProductCode": "ES",
  "YearMonth": {
    "$date": "1998-03-01T00:00:00.000Z"
  },
  "Exchange": "XCME",
  "Name": "E-Mini S&P 500 Future - Mar 1998",
  "ExpirationDate": {
    "$date": "1998-03-20T00:00:00.000Z"
  },
  "Notes": "1 contract -> $50 x S&P 500 Index"
}

The exception is thrown at ToListAsync(). Why I cannot use an existing property has discriminator with new driver? How to fix that?