Getting systemd-nspawn to work with my ruleset

[Kevin P <petrilli.kevin@xxxxxxxxx>]

Good afternoon everyone
I've been trying to work this out for over a week but I really don't
have a clue past this point.
My notebook runs Arch Linux, running systemd-networkd as network
manager and (trying to) have systemd-nspawn
to work for containers.
I have a few rules in  /etc/nftables.conf :

table inet filter {
    chain input {
        type filter hook input priority filter; policy drop;
        ct state { established, related } accept
        ct state invalid drop
        ip protocol icmp accept
        meta l4proto ipv6-icmp accept
        udp dport 5353 accept
        reject
    }

    chain forward {
        type filter hook forward priority filter; policy accept;
    }

    chain output {
    }
}

That somehow conflict with those added by systemd-nspawn, after I
start a container with -n option:

table ip io.systemd.nat {
    set masq_saddr {
        type ipv4_addr
        flags interval
        elements = { 192.168.240.176/28 }
    }

    map map_port_ipport {
        type inet_proto . inet_service : ipv4_addr . inet_service
    }

    chain prerouting {
        type nat hook prerouting priority dstnat + 1; policy accept;
        fib daddr type local dnat ip to meta l4proto . th dport map
@map_port_ipport
    }

    chain output {
        type nat hook output priority -99; policy accept;
        ip daddr != 127.0.0.0/8 oif "lo" dnat ip to meta l4proto . th
dport map @map_port_ipport
    }

    chain postrouting {
        type nat hook postrouting priority srcnat + 1; policy accept;
        ip saddr @masq_saddr masquerade
    }
}

Even though I fail to understand how. If, before starting a container,
I flush my ruleset, the networking works as expected within
the container, but if I don't, and my rules are active, I get no
internet connection for the containers.
Any help explaining why and how to fix this problem would be MUCH appreciated.
Thanks

Kevin P.