Quick reproduction in LXD:
lxc launch ubuntu:18.04 x lxc exec x apt update lxc exec x apt install iptables-persistent lxc exec x netfilter-persistent save
Quick reproduction in LXD:
lxc launch ubuntu:18.04 x persistent save
lxc exec x apt update
lxc exec x apt install iptables-persistent
lxc exec x netfilter-