MS DTC and parallel query execution exception - Transaction Coordinator (MS DTC) transaction must be committed by remote client

In our solution we have section like this:

           using (var transaction = new TransactionScope(TransactionScopeAsyncFlowOption.Enabled))
           {
               var task1 = _repository1.SaveAsync(dto);
               var task2 = _repository2.SaveAsync(dto1);

               await Task.WhenAll(task1 , task2).ConfigureAwait(false);

               transaction.Complete();
           }

Each of the repo creates a new DbContext with new connection

And from time to time we have MS DTC error:

System.Data.SqlClient.SqlException: Current Microsoft Distributed Transaction Coordinator (MS DTC) transaction must be committed by remote client.
at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)\r\n   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)\r\n   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)\r\n   at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj)

MS DTC is configured properly, PS command "Test-Dtc -LocalComputerName "$env:COMPUTERNAME" -Verbose" returns:

VERBOSE: ": Firewall rule for "RPC Endpoint Mapper" is enabled."
VERBOSE: ": Firewall rule for "DTC incoming connections" is enabled."
VERBOSE: ": Firewall rule for "DTC outgoing connections" is enabled."
VERBOSE: server: AuthenticationLevel: NoAuth
VERBOSE: server: InboundTransactionsEnabled: True
VERBOSE: server: OutboundTransactionsEnabled: True
VERBOSE: server: RemoteClientAccessEnabled: True
VERBOSE: server: RemoteAdministrationAccessEnabled: True
VERBOSE: server: XATransactionsEnabled: True
VERBOSE: server: LUTransactionsEnabled: True

I found similar topic - TransactionScope() and parallel query execution, unfortunately, there is a no explanation of this error, and no solution. I don't have enough score to ask author, did he find a solution and what was a real issue.

Of course I can change the logic to:

           using (var transaction = new TransactionScope(TransactionScopeAsyncFlowOption.Enabled))
           {
               await _repository1.SaveAsync(dto).ConfigureAwait(false);
               await _repository2.SaveAsync(dto1).ConfigureAwait(false);

               transaction.Complete();
           }

But I want to understand, what is the reason, why MS DTC complains when asynchronous execution is used in TransactionScope