C# driver ver 2.21.0.0 System.ArgumentException - regression from driver 2.18.0

Hi,
I have that query:

EFeatureType flTypes = { EFeatureType.TraumaCad, EFeatureType.TraumaCadSolve };
var query = DbGeneralContext.FeatureTypes.AsQueryable()
.Where(a => flTypes.Contains(a.FeatureTypeId))
.GroupJoin(DbGeneralContext.FeatureLicenses,
ft => ft.Id,
fl => fl.FeatureTypeId,
(ft, fl) => new { FT = ft, FL = fl })
.SelectMany(a => a.FL,
(a, fl) =>
new
{
fl.AccountId,
a.FT.FeatureTypeId,
fl.AssignedToId,
})
.GroupBy(p => p.AccountId)
.Select(g => new
{
AccountId = g.Key,
TotalTraumaCadLicenses = g.Sum(s => s.FeatureTypeId == EFeatureType.TraumaCad ? 1 : 0),
AssignedTraumaCadLicenses =
g.Sum(s => s.FeatureTypeId == EFeatureType.TraumaCad && s.AssignedToId.HasValue ? 1 : 0),
TotalTraumaCadMobileLicenses = g.Sum(s => s.FeatureTypeId == EFeatureType.TraumaCadSolve ? 1 : 0),
AssignedTraumaCadMobileLicenses =
g.Sum(s => s.FeatureTypeId == EFeatureType.TraumaCadSolve && s.AssignedToId.HasValue ? 1 : 0)
});
try
{
var res = await query.ToListAsync();
catch (Exception ex)
{
// handle exception
}
}

explain:
DbGeneralContext.FeatureTypes is IMongoCollection
DbGeneralContext.FeatureLicenses is IMongoCollection
FeatureTypeDal has a field: public EFeatureType FeatureTypeId { get; set; } // which is enum

under driver 2.18.0 the query runs well
under version 2.21.0.0 the following exception occur:
System.ArgumentException: Value type of serializer is VoyantHealth.Common.Data.Account.EFeatureType and does not match member type System.Int32.
Parameter name: serializer
at MongoDB.Bson.Serialization.BsonMemberMap.SetSerializer(IBsonSerializer serializer)
at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.MemberInitExpressionToAggregationExpressionTranslator.Translate(TranslationContext context, Expression expression, NewExpression newExpression, IReadOnlyList1 bindings) at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(TranslationContext context, LambdaExpression lambdaExpression, IBsonSerializer parameterSerializer, Boolean asRoot) at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.SelectMethodToPipelineTranslator.Translate(TranslationContext context, MethodCallExpression expression) at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToExecutableQueryTranslators.ExpressionToExecutableQueryTranslator.Translate[TDocument,TOutput](MongoQueryProvider1 provider, Expression expression)
at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery2.ToCursorAsync(CancellationToken cancellationToken) at MongoDB.Driver.IAsyncCursorSourceExtensions.<ToListAsync>d__161.MoveNext()
— End of stack trace from previous location where exception was thrown —
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at VoyantWebCloud.DataAccess.Mongo.Reg.IO.FeatureLicenseDalIo.d__93.MoveNext() in D:\Repos\apps-quentry-services\Data\VoyantWebCloud.DataAccess.Mongo.Reg\IO\FeatureLicenseDalIo.cs:line 1271

Any suggestions to solve that?
Thanks,
Itzhak