@Gargron @CobaltVelvet Linux has a very weird way of dealing with IPv6. One of the extensions of this is that for every barebones network tool out there exist two separate versions, one of them meant to work with IPv4 (without any indicator) and one of them with IPv6 (usually indicated by a "6" or v6" in the name).

I presume @CobaltVelvet had been using "just" the regular "iptables" (meant for managing the internal firewall by the Linux kernel) but not also "ip6tables" when hardening the setup