Comment 7 for bug 1641236

I've been able to re-create this using fresh install of Ubuntu 18.04 without using LXC or LXD, but just using network namespaces.

Setup 2 namespaces with IPVLAN:

ip netns add ns1
ip link add name ipv1 link enp0s3 type ipvlan mode l3s
ip link set dev ipv1 netns ns1
ip netns exec ns1 ip addr add 10.1.20.252/32 dev ipv1
ip netns exec ns1 ip link set ipv1 up
ip netns exec ns1 ip link set lo up
ip netns exec ns1 ip -4 r add default dev ipv1

ip netns add ns2
ip link add name ipv2 link enp0s3 type ipvlan mode l3s
ip link set dev ipv2 netns ns2
ip netns exec ns2 ip addr add 10.1.20.253/32 dev ipv2
ip netns exec ns2 ip link set ipv2 up
ip netns exec ns2 ip link set lo up
ip netns exec ns2 ip -4 r add default dev ipv2

Enter namespace 1 and start a ping to other namespace:

sudo ip netns exec ns1 ping 10.1.20.253

Then run tcpdump in namespace 2 listening for all packets without DNS resolution:

sudo ip netns exec ns2 tcpdump -i any -nn

This doesn't output any captured packets.

However running tcpdump with -l (Make stdout line buffered) does help:

sudo ip netns exec ns2 tcpdump -i any -nn -l