IOCP when read is busy?

Question

while I am writing some IOCP server-client codes, I saw misc behavior in IOCP.

The scenario goes here,

1. Register Socket to IOCP
2. Recv event catched by GetQueuedCompletionStatus

    while (!mExitFlag)
    {
        bool bSuccess = ::GetQueuedCompletionStatus(mIocpHandle, &dwIoSize, (PULONG_PTR)&client, (LPOVERLAPPED*)&ioData, INFINITE);
        
        logger->st = std::chrono::steady_clock::now();

        // ... queue to recv worker
    }

3. then read buffer (char[]) related to iocp registered buffer (WSABUF)

        int dataLength = recvBytes; // when iocp completed
        int pktLength = Serializer::toInt32(mBuffer + mDataPos);
    
        if (dataLength > 0 && pktLength == 0)
        {
            using namespace std::chrono;

            char buffer[512];
            if (mBuffer[mDataPos] == 0)
            {
                // take snapshot
                memcpy(buffer, &mBuffer[mDataPos], dataLength);
            }

            while (mBuffer[mDataPos] == 0) { }
            
            // elapsed < 1ms 
            auto elapsed_in_microseconds = CTimer::count<microseconds>(mLogger->st);
            
            printf("elapsed %llu us", elapsed_in_microseconds);
            int val = mBuffer[mDataPos]; // this gives positive value

            throw std::runtime_error("serializer failed to read packet length");
        }

Snapshot in buffer[512] gives always 0 padded with dataLength.
After some microseconds elapsed, the mBuffer (WSABUF registered) is retrieved with data.
I checked the recv pending and handling in the single thread with log.

• I observed this only happens when client sends huge data in shortly.
• When the client sends data with term (10ms?), it was fine.

Does anyoneknows this IOCP issue?
Perhaps the solution can be waiting the buffer when client recv buffer is busy.