In channels.c:
channel_input_status_confirm
channel_lookup()
sees channel type is X11_LISTENER, so issues:
logit("Non-public channel %d, type %d.", id, c-type);
return null channel
issue logit("channel_input_success_failure: %d: unknown", remote_id);
The mystery (to me) is why this is done for the second (and possibly
each successive port forwarded session) but not for the first.