During session load, all earlier configure_processors() calls
were useless and not taking I/O into account! The Delivery ::pan_out()
needs _output I/O ports in order to correctly report the correct
port-count!
This worked in earlier versions because Config->map_parameters()
triggered [two] processor re-configs via ::listen_position_changed().
That behavior was changed in 1af123465c
b890cf73ad