Really? I wouldn't have expected that: The listen PCBs always are
listen PCBs. You can listen on as many TCP ports as you have listen
PCBs. What happens if a client connects is a new (standard) PCB is
allocated and passed to your application. With a port scan, I'd
expect you'd run out of standard PCBs eventually and no new PCBs will
be available. This might also fit to what you are experiencing, for
example you cannot connect to your device with a web browser after a
port scan.
However, TCP has timeout mechanisms, and once they regard the
half-open connections (I suspect the port scan utility leaves them
half-open) as timed out, they will be deallocated and available for
new connections again.