SSH disconnections

Lately certain ssh connections have been dropping on a Pi4 running -current
as of yesterday. The odd thing is that out of several active ssh connections
with that host, only one, running a cu session, is failing. The termination
message is
login: packet_write_wait: Connection to 192.168.1.11 port 22: Broken pipe

/var/log/aculog contains lines similar to 
bob (Wed Oct  6 23:39:41 2021) <cu115200, , /dev/cuaU0> call completed
but the timestamp predates, at least sometimes, the actual disconnection.

The host originating the connection is also a Pi4, running RasPiOS that's
up-to-date as of a few days ago. There are numerous other active ssh
connections to Pi2's and Pi3's, all of which remain connected and running
for days, sometimes weeks. 

It looks as if the combination of cu and ssh is required to provoke the
problem, any suggestions on where to look appreciated!

Thanks for reading,

bob prohaska