What could cause a non-blocking socket to block on `recv`?

I have a TCP/IP socket set to non-blocking that is blocking anyway. The socket is only referenced in one thread. This code works on Windows (with a few call substitutions) but not on Linux. I have code that looks like this (Don't mind the C-style casts -- this was written long ago. Also, I trimmed it up a bit, so let me know if I accidentally trimmed off a step. Chances are that I'm actually doing that step. The actual code is on another computer, so I can't copy-paste.):

// In the real code, these are class members. I'm not bonkers
int mSocket;
sockaddr_in mAddress;

void CreateSocket(
    unsigned int ipAddress,
    unsigned short port)
{        
    // Omitting my error checking in this question for brevity because everything comes back valid
    mSocket = socket(AF_INET, SOCK_STREAM, 0);  // Not -1

    int oldFlags = fctnl(mSocket, F_GETFL, 0);  // Not -1
    fcntl(mSocket, F_SETFL, oldFlags | O_NONBLOCK);  // Not -1

    mAddress.sin_family = AF_INET;
    mAddress.sin_addr.s_addr = ipAddress;  // address is valid
    mAddress.sin_port = htons((u_short)port);  // port is not 0 and allowed on firewall
    memset(mAddress.sin_zero, 0, sizeof(mAddress.sin_zero));

    // <Connect attempt loop starts here>
    connect(mSocket, (sockaddr*)&mAddress, sizeof(mAddress));  // Not -1 to exit loop
    // <Connect attempt loop ends here>
    // Connection is now successful ('connect' returned a value other than -1)
}

// ... Stuff happens ...

// ... Then this is called because 'select' call shows read data available ...
void AttemptReceive(
    MyReturnBufferTypeThatsNotImportant &returnedBytes)
{
    // Read socket
    const size_t bufferSize = 4096;
    char buffer[bufferSize];
    int result = 0;

    do {
        // Debugging code: sanity checks
        int socketFlags = fcntl(mSocket, F_GETFL, 0);  // Not -1
        printf("result=%d\n", result);
        printf("O_NONBLOCK? %d\n", socketFlags & O_NONBLOCK);  // Always prints "O_NONBLOCK? 2048"

        result = recv(mSocket, buffer, bufferSize, 0);  // NEVER -1 or 0 after hundreds to thousands of calls, then suddenly blocks

        // ... Save off and package read data into user format for output to caller ...
    } while (result == bufferSize);
}

I believe, because AttemptReceive is called in response to select, that the socket just happens to contain exactly a number of bytes equal to a multiple of the buffer size (4096). I've somewhat confirmed this with the printf statements, so it never blocks on the first loop-through. Every time this bug happens, the last two lines to get printed before the thread blocks are:

result=4096
O_NONBLOCK? 2048

Changing the recv line to recv(mSocket, buffer, bufferSize, MSG_DONTWAIT); actually "fixes" the issue (suddenly, recv occasionally returns -1 with errno EWOULDBLOCK/EAGAIN (both equal to each other on my OS)), but I'm afraid I'm just putting a band-aid on a gushing wound, so to speak. Any ideas?

P.S. the address is "localhost", but I don't think it matters.

Note: I'm using an old compiler (not by choice), g++ 4.4.7-23 from 2010. That may have something to do with the issue.