With Amount defined in the class as:

[BsonIgnoreIfNull]
public decimal? Amount { get; set; } = null;

The following update returns throws the error: One or more errors occurred. (Unable to determine the serialization information for x => value(i3vDB.CollectionsTrans.Transaction).MerchantAmount.)

      await myCollection.UpdateOneAsync(x => x.Id == id,
Builders<Transaction>.Update.Set(x => Amount, newValue)).ConfigureAwait(false);

I am able to use the same logic to update bools, and other data types without issues.  I've tried with and without setting the BsonSerializer to support the new decimal format:

    BsonSerializer.RegisterSerializer(typeof(decimal), new DecimalSerializer(BsonType.Decimal128));
BsonSerializer.RegisterSerializer(typeof(decimal?), new NullableSerializer<decimal>(new DecimalSerializer(BsonType.Decimal128)));

I am able to load and save the object normally (.Find and .ReplaceOneAsync)

Full error message:

System.AggregateException : One or more errors occurred. (Unable to determine the serialization information for x => value(i3vDB.CollectionsTrans.Transaction).MerchantAmount.)
----> System.InvalidOperationException : Unable to determine the serialization information for x => value(i3vDB.CollectionsTrans.Transaction).MerchantAmount.
at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
at System.Threading.Tasks.Task.Wait()
at Velocity.Common.Test.DynamicPayout.CalculateJournalEntries_CreditCardSale.CreditCardSale_Refund_Calculate(DateTime transactionDateTime, DateTime expectedDate) in C:\dbs\chargepayment\Blocks\Velocity\Common.Test\DynamicPayout\CalculateJournalEntries_CreditCardSale.cs:line 571
--InvalidOperationException
at MongoDB.Driver.ExpressionFieldDefinition`2.Render(IBsonSerializer`1 documentSerializer, IBsonSerializerRegistry serializerRegistry, Boolean allowScalarValueForArrayField)
at MongoDB.Driver.OperatorUpdateDefinition`2.Render(IBsonSerializer`1 documentSerializer, IBsonSerializerRegistry serializerRegistry)
at MongoDB.Driver.MongoCollectionImpl`1.ConvertWriteModelToWriteRequest(WriteModel`1 model, Int32 index)
at System.Linq.Enumerable.SelectIterator[TSource,TResult](IEnumerable`1 source, Func`3 selector)+MoveNext()
at System.Collections.Generic.List`1.AddEnumerable(IEnumerable`1 enumerable)
at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
at MongoDB.Driver.Core.Operations.BulkMixedWriteOperation..ctor(CollectionNamespace collectionNamespace, IEnumerable`1 requests, MessageEncoderSettings messageEncoderSettings)
at MongoDB.Driver.MongoCollectionImpl`1.CreateBulkWriteOperation(IEnumerable`1 requests, BulkWriteOptions options)
at MongoDB.Driver.MongoCollectionImpl`1.BulkWriteAsync(IClientSessionHandle session, IEnumerable`1 requests, BulkWriteOptions options, CancellationToken cancellationToken)
at MongoDB.Driver.MongoCollectionImpl`1.UsingImplicitSessionAsync[TResult](Func`2 funcAsync, CancellationToken cancellationToken)
at MongoDB.Driver.MongoCollectionBase`1.UpdateOneAsync(FilterDefinition`1 filter, UpdateDefinition`1 update, UpdateOptions options, Func`3 bulkWriteAsync)