Stable/14 dropping ssh connections to FT232 usb-serial adapter

A stable/14 install on a Pi2 v1.1 (armv7) host seems to
work quite well, apart from dropping ssh connections when
used to run a cu or tip session to an FT232 usb-serial
adapter. Ssh connections to interactive shells seem to
remain up indefinitely, but drop within a couple of hours
(between the same hosts) when used to run cu or tip.

The only debug info provided by ssh looks like:

login: Sep 29 08:25:19 pelorus sshd[53926]: error: PAM: Authentication error for illegal user username from 179.63.55.224
Sep 29 08:31:52 pelorus sshd[53966]: error: Fssh_kex_exchange_identification: Connection closed by remote host
[....]
Sep 29 08:41:05 pelorus sshd[54026]: error: PAM: Authentication error for illegal user ubnt from 103.146.55.222


FreeBSD/arm64 (pelorus.zefox.org) (ttyu0)

login: debug3: send packet: type 1
client_loop: send disconnect: Broken pipe
bob@raspberrypi:~ 

Since the problem doesn't happen when running a shell, it's tempting
to think this is caued by tip, cu or maybe the driver for the adapter. 
It seems to be confined to stable/14, neither current nor 12.4 has
this tendency. The stable/14 host is up to date as of yesterday.

This sort of trouble has been present for some time on -current
for both Pi2 (armv7) and Pi3 (aarch64), but it seems to have gone
away on aarch64, which now holds ssh-to-ft232 sessions indefinitely.

Thanks for reading, and any hints.

bob prohaska
.