firewall – IP forwarding does not work

I have 2 Ubuntu systems , both have Ubuntu 20.04, one has server (5.4.0-42-generic) , the other has desktop (5.4.0-58-generic).
I can get ip forwarding working on one system which has the server .
But I can’t get it to work on the other system which has desktop.
The network configuration is the same on the system which is able to forwards packets and the one which does not.

I have checked that

net.ipv4.ip_forward = 1

net.ipv6.conf.all.forwarding = 1

I have flushed iptables using
iptables -F

ufw is inactive

I can ping both interfaces of the router from either side, so I think the routing table on the hosts on either side is correct.
I can also ping the interfaces on the hosts on either side from the router, so I think the routing table on the router is correct.
I have disabled NetworkManager; am using networkd in /etc/netplan/*yaml

Any suggestions on how to debug this ?