ESP8266 tcp recv returning errno 11 (EAGAIN) when handling large amounts of data - tcp

Question

I am running ESP8266_RTOS_SDK_3.4 for an app that uses TCP to talk to a private port on a local server. In normal operation, it uploads large amounts of data to the server and receives acknowledgements that are usually less than 200 bytes: this always works fine. When I do an OTA update, where it's mainly receiving data, TCP recv fails on attempting to read the first block, and errno is set to 11 (EAGAIN). Even if I make the server send just 1024 bytes, the same thing happens.
This is the TCP connect and recv code:
bool net_tcp_connect (SENDER_DESTINATION * dest) {
struct sockaddr_in destAddr;
if (!find_host (dest->hostname)) {
return false;
}
memset(&destAddr, 0, sizeof(destAddr));
memcpy (&destAddr.sin_addr, findhost_ip (), sizeof (destAddr.sin_addr));
destAddr.sin_family = AF_INET;
destAddr.sin_port = htons (dest->port);
sock = socket(AF_INET, SOCK_STREAM, 0);
if (sock < 0) {
LOGEF("Create: errno %d", errno);
return false;
}
struct timeval tv;
tv.tv_sec = dest->timeout;
tv.tv_usec = 0;
setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, (const char*)&tv, sizeof(tv));
if (connect(sock, (struct sockaddr *)&destAddr, sizeof(destAddr)) != 0) {
LOGEF("Connect: %s %d errno %d", findhost_str (), dest->port, errno);
EVENT_HERE ( );
net_tcp_close ();
return false;
}
return true;
}
// --------------------------------------------------------------------------------------------
int net_tcp_recv (void * buffer, int max_length) {
if (sock < 0)
return false;
int bytes_received = recv (sock, buffer, max_length, 0);
if (bytes_received < 0) {
LOGEF("Receive: errno= %d", errno);
net_tcp_close ();
bytes_received = 0;
}
return bytes_received;
}
EAGAIN can be a sign of a receive timeout, but the timeout is set to 30 seconds and the server usually sends out the first 32k bytes in less than a second.
The ESP8266 code does run OK on some access points and, as far as I can tell, the same code on an ESP32 runs OK on all access points.
Any suggestions for why this might happen, or things that I could try changing in the code or the ESP setup to make it work reliably on any access point?