The RST (Reset) is sent to inform the client that the data it sent was not read by the server. The RST avoids here the 4-way handshake for the TCP connection closure and the long wait times, if the client doesn't behave normal.
For the case here the server should call shutdown with SHUT_WR after sending the data and then drain the incoming data before closing the socket.
For the case here the server should call shutdown with SHUT_WR after sending the data and then drain the incoming data before closing the socket.