Using tcpdump with a filter, on the loopback interface
doesn't match any packets.
Using -i lo, without a filter, or -i ppp0 port 23 does work,
returing the requested packets, but
~# /usr/sbin/tcpdump -i lo port 23
doesn't (using src or dst rather that port doesn't work
Fixed in tcpdump-3.4-11. Thanks for the report.