-current dropping ssh connections

I've got a Pi4 running -current that seems to selectively drop ssh connections.

Connections running a shell seem to stay up, but a session running tip to a
usb-serial adapter (FTDI TTL232R-3V3) seems go away within a few hours. 
There don't seem to be any error messages on the console at all, the client 
session simply reports 
client_loop: send disconnect: Broken pipe

Searches through /var/log/sshd_debug.log find many transactions between
the ssh client and the -current target host, but none seem to be error
messages; all are either connection reports or disconnects by user.

This sort of behavior has been intermittent with aarch64 among both
the Pi4 and a pair of Pi3s for some time, but now only the Pi4 is
dropping connections. 

I've tried searching /var/log/sshd_debug.log for the keywords tip,
ucom, the IP address of the NAT client used to connect and cuaU0.
Are there other things worth looking for?

Right now I'm using in /etc/rc.conf the line
sshd_flags="-E /var/log/sshd_debug.log"
which is already quite verbose. Is there a better
option that emphasizes errors over normal traffic?

Thanks for reading,

bob prohaska