emscripten_fetch on bg thread not working

[audio boy]

Ive recently started using emscripten and have been stuck for several days trying to perform HTTP requests on threads.  i originally tried calling to js xhr and then fetch apis, but couldnt get that to work, so then tried the emscripten_fetch api. 

my build flags
-std=c++20
-pthread
-s USE_PTHREADS=1
-s ALLOW_BLOCKING_ON_MAIN_THREAD=1
-s PTHREAD_POOL_SIZE=8
-s FETCH=1

Below is an isolated code example which hangs on emscripten_fetch.  the code is identical to the example on official docs (plus a short timeout specified but that makes no difference).  if i just do a long emscripten_sleep it works ok, so threads in general appear to be ok.

Can anyone shed any light on this?
Thanks

---
#include <emscripten.h>
#include <emscripten/fetch.h>
#include <thread>
#include <chrono>

static_assert(__EMSCRIPTEN_PTHREADS__ , "no threads");

void Print(const char *str)
{
    emscripten_log(EM_LOG_CONSOLE, "%s", str);
}

int main()
{
Print("main");

std::atomic <bool> completed = false;

std::thread task([&completed]()
{
Print("http on bg thread");

emscripten_fetch_attr_t attr;
emscripten_fetch_attr_init(&attr);
strcpy(attr.requestMethod, "GET");
attr.timeoutMSecs = 1;
attr.attributes = EMSCRIPTEN_FETCH_LOAD_TO_MEMORY | EMSCRIPTEN_FETCH_SYNCHRONOUS;
emscripten_fetch_t *fetch = emscripten_fetch(&attr, "testfile.json"); // Blocks here until the operation is complete.
if (fetch->status == 200)
{
Print("downloaded ok");
}
else
{
Print("download failed");
}
 
emscripten_fetch_close(fetch);

completed.store(true);

Print("/http on bg thread");
});

while (!completed.load())
{
Print("waiting for bg thread");

emscripten_sleep(50);
}

task.join();

Print("/main");

return 0;
}