The Linux box instantly turns into a router as soon as you run `sysctl net.ipv4.ip_forward=1`, because the default policy for FORWARD table is ACCEPT.
You need to explicitly reconfigure the iptables/nftables to prevent that from happening.
Some software, say LXD/Incus, enable forwarding automatically upon installation/startup, and do not configure firewall to block non-their traffic, making the machine an open router. I've reported that, the developers said that's by design (despite other virtualization/containerization systems block forwarding if they happen to enable the sysctl).
You need to explicitly reconfigure the iptables/nftables to prevent that from happening.
Some software, say LXD/Incus, enable forwarding automatically upon installation/startup, and do not configure firewall to block non-their traffic, making the machine an open router. I've reported that, the developers said that's by design (despite other virtualization/containerization systems block forwarding if they happen to enable the sysctl).