Cannot Ethernet tunnel with ssh on NetBSD 9

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I have two ends both with recent netbsd-9 and using base ssh/sshd:

OpenSSH_8.0 NetBSD_Secure_Shell-20190418-hpn13v14-lpk, OpenSSL 1.1.1g  21 Apr 2020

ssh for an interactive session works fine (and for things like rsync), but 
as soon as I try to use L2 tunneling, it quickly dies with errors like:

sshd[1373]: fatal: channel 0: adjust 98329 overflows remote window 4294871838

Client-side I am using:
ssh -o Tunnel=ethernet -w 0:0 remotehost

Server-side sshd_config contains:
PermitTunnel ethernet

tap0 is a member of bridge0 as is a local LAN interface.

I think this is perhaps related to simply the amount of packets queued to 
send over the tunnel. There are two LAN interfaces, one has an IP 
address and is providing services such as web-proxy, the other is 
used solely with vlan). Bridging to the quieter interface will run for 
around 30s whereas the busy interface fails within a second.

--
Stephen