ok this is what normally happens every x amount of seconds the server pings you, the client. In a normal situation, mIRC notices this and pongs back. When you have pinged out, mIRC notices that it hasnt recieved a ping event and i think tries to ping the server itself (i might be wrong but im pretty sure) and waits on the server to pong. Remeber this is after like x amount of time of no activity on the connection. Aty this point if the server does not pong back, mIRC assumes you are disconnected and should disconnect. Why it didnt for you beats me. But the process seems to work fine on my end.
