UDP receive first 4 letters missing

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i use visual studio 2019 community.
and UnrealEngine ver.4.25.4

goal is receive UDP from other program
maybe 10~20time pre second

in my test, 1 per second, something like
@anonymous_user_142fbdd5,1,35.12345,127.123456,0,0,0,0,0,0,0,0,0,0
identifier(for thins udp), index of target actor, gps lat, gps lng

and then when i see print log, there are like
,35.12345,127.123456,0,0,0,0,0,0,0,0,0,

@anonymous_user_142fbdd5,1 <-this part is missing

what makes me crazy is. sometime i restart whole Epic Games Launcher, it worked about 10seconds and have same issue.

here are codes

UDPRecive.cpp

bool AUDPSend::StartUDPSender(const FString & YourChosenSocketName, const FString & TheIP, const int32 
ThePort, bool UDP)
{
RemoteAddr = ISocketSubsystem::Get(PLATFORM_SOCKETSUBSYSTEM)->CreateInternetAddr();
bool bIsValid;
RemoteAddr->SetIp(*TheIP, bIsValid);
RemoteAddr->SetBroadcastAddress();
RemoteAddr->SetPort(ThePort);
if (!bIsValid)
{
	return false;
}

SenderSocket = FUdpSocketBuilder(*YourChosenSocketName)
	.AsReusable()// enables the bound address to be reused by other sockets
	.WithBroadcast()
	.WithSendBufferSize(2 * 1024 * 1024)
	;

check(SenderSocket->GetSocketType() == SOCKTYPE_Datagram);
int32 SendSize = 2 * 1024 * 1024;
SenderSocket->SetSendBufferSize(SendSize, SendSize);
SenderSocket->SetReceiveBufferSize(SendSize, SendSize);
if (bIsValid)
{
	bIsValid = true;
}
return bIsValid;

}

void AUDPRecive::DataRecv(FString & str, bool & success)
{
	if (!ListenSocket)
	{
		ScreenMsg("No sender socket");
		success = false;
	}
	TSharedRef<FInternetAddr> targetAddr = ISocketSubsystem::Get(PLATFORM_SOCKETSUBSYSTEM)->CreateInternetAddr();
	TArray<uint8> ReceivedData;//Define a receiver
	uint32 Size;
	if (ListenSocket->HasPendingData(Size))
	{
		success = true;
		str = "";
		uint8 *Recv = new uint8[Size];
		int32 BytesRead = 0;
		// Adjust the array to a given number of elements. The new element will be initialized.
		ReceivedData.SetNumUninitialized(FMath::Min(Size, 65507u));
		ListenSocket->RecvFrom(ReceivedData.GetData(), ReceivedData.Num(), BytesRead, *targetAddr);
		char ansiiData[1024];
		FMemory::Memcpy(ansiiData, ReceivedData.GetData(), BytesRead);//Copy data to the receiver
		ansiiData[BytesRead] = 0; //Judge the end of the data
		FString debugData = ANSI_TO_TCHAR(ansiiData); //string conversion
		str = debugData;
	}
	else
	{
		success = false;  
	}

}

void AUDPRecive::Recvd(const FArrayReaderPtr& ArrayReaderPtr, const FIPv4Endpoint& EndPt)
{
	ScreenMsg("Received bytes", ArrayReaderPtr->Num());

	uint8 data[512];
	FMemory::Memzero(data, 512);

	FMemory::Memcpy(data, ArrayReaderPtr->GetData(), ArrayReaderPtr->Num());
	FString ReceivedData = ((const char*)data);

	AsyncTask(ENamedThreads::GameThread, [&]() {
		BPEvent_DataReceived(ReceivedData);
	});
}

i tried ANSI_TO_TCHAR to UFT8_TO_TCHAR
it didn’t work.

and sometime when i tried close editer before stop UDP sender.
unreal engine crash by memory issue.

anyone help me slove this problem? now i’m thinking remove @letter.