Why cosocket read timeout error when receive response on time?

i would like to send data to server, and receive process result, but i got some lua tcp socket read timed out error,
The error is as follows:

2021/07/19 04:05:33 [error] 134162#0: *383692285 lua tcp socket read timed out, client: 183.84.4.102, server: _, request: "GET /ping HTTP/1.1", 
2021/07/19 04:05:56 [error] 134166#0: *383066134 lua tcp socket read timed out, client: 111.199.189.186, server: _, request: "POST /v1/client/heartbeat HTTP/1.1", 
2021/07/19 04:06:57 [error] 134166#0: *383767683 lua tcp socket read timed out, client: 124.126.141.88, server: _, request: "POST /desktop/v1/client/heartbeat HTTP/1.1", 
2021/07/19 04:08:45 [error] 134166#0: *383850419 lua tcp socket read timed out, client: 122.224.227.210, server: _, request: "GET /cs/message/latest HTTP/1.1", 
--More--

my code is as follows, which is called in access_by_lua_block:

function Connector._query_tcp(self, packed_data)
    local sock = ngx.socket.tcp()
    sock:settimeouts(15, 5, 20)

    local ok, err = sock:connect(self.server, self.port)
    if not ok then
        return nil, err, "connect"
    end

    local ok, err = sock:send(packed_data..self.boundary)
    if not ok then
        return nil, err, "send"
    end

    local resp_data, err = sock:receiveuntil(self.boundary)()
    if not resp_data then
        return nil, err, "read"
    end
    ok, err = sock:setkeepalive(60000, 5000)
    return resp_data, nil, ""
end

other setting of nginx:
tcp_nodelay on;

i use tcpdump to capture the packets, and use wireshark to find the timeout reqs with reqid. From the result, i find my network card receive process result from server immediately, the timecost is less then 2 milliseconds, my settings of read timeout is 20 milliseconds, this confuse me much

Any help is precious!

Start with the cosocket timeout behavior around ngx.socket.tcp(), settimeouts(), and receiveuntil(), then compare it with the access_by_lua_block example and the tcpdump/Wireshark timestamps provided. Done means determining whether the reported timing is expected behavior or a reproducible module bug, with a focused reproduction if applicable.