Comment 5 for bug 1863873

Just in case, I also removed any arptables or iptables on the host. The problem is still present.
Here is an even simpler LXC configuration file to reproduce (privileged LXC container on Debian Buster, 5.4.13 kernel):

# Distribution configuration
lxc.include = /usr/share/lxc/config/common.conf
lxc.arch = linux64

# Container specific configuration
lxc.hook.pre-start = /usr/local/share/lxc/hooks/pre-start.sh vps526706
lxc.hook.post-stop = /usr/local/share/lxc/hooks/post-stop.sh vps526706
lxc.hook.destroy = /usr/local/share/lxc/hooks/destroy.sh vps526706
lxc.mount.fstab = /lxc/vps526706/fstab
lxc.rootfs.path = dir:/lxc/vps526706/rootfs
lxc.uts.name = vps526706

# Network configuration
lxc.net.0.type = veth
lxc.net.0.veth.pair = vps526706
lxc.net.0.flags = up
lxc.net.0.link = br0
lxc.net.0.hwaddr = 02:00:00:52:67:06
lxc.net.0.name = eth0
lxc.net.0.ipv4.gateway = 192.168.252.1
lxc.net.0.ipv4.address = 192.168.252.171/32