Unexplained port blocking

I'm having an issue with Ubuntu 21.04, where I'm unable to access ports that are behind Port Fowarding/DNAT on my router.

I have a working port forward on my router from (Public IP):443 to 192.168.2.223:4443

I know that this works, because I have an older system that when I plug back in and configure with 192.168.2.223, I can jump on to https://(Public IP) using an external jumpbox and I can see a webpage.

However the newer system I have here refuses the connection when attempting to access it via the port forward, but I can jump on to https://192.168.2.223:4443 and voila it pops up.

I've enabled IP Forwarding, have removed apparmour, removed UFW, cleared IPTables...and have now run out of ideas. I'm sure it's something really silly and minor somewhere that I've overlooked, and I'm hoping some fresh eyes will be able to spot it and point me in the right direction.

Any help appreciated!

Extra Notes: I can see the NAT translation occur correctly, and the traffic is directed to the new Ubuntu box -- and there is nothing in the service logs on the Ubuntu server, it's like the traffic gets to the new Ubuntu system and it just disregards it.