I agree with dseven. Run a packet capture on the WAN. Assuming you see them, then alter your one to one NAT rule to log. Also create a rule on the LAN side to permit logging of packets to the Debian 12 box.
You do mention that even with the one to one in place, you are getting the wrong IP for outbound packets, so it's possible the packets are making it all the way to the Debian box, but the replies to the TCP handshake come from the .105 ip instead of .106. A packet capture on the Debian 12 box could be informative too.
I too have a /29. I've only done port forwards, not 1 to 1, so I can't help with settings there.
I will say you should do more to narrow down in what point in the process are things breaking. With that info, others that have worked with 1 to 1 NATs may have some insight.
Is it possible something on the Debian firewall is permitting LAN but not non-LAN?
You do mention that even with the one to one in place, you are getting the wrong IP for outbound packets, so it's possible the packets are making it all the way to the Debian box, but the replies to the TCP handshake come from the .105 ip instead of .106. A packet capture on the Debian 12 box could be informative too.
I too have a /29. I've only done port forwards, not 1 to 1, so I can't help with settings there.
I will say you should do more to narrow down in what point in the process are things breaking. With that info, others that have worked with 1 to 1 NATs may have some insight.
Is it possible something on the Debian firewall is permitting LAN but not non-LAN?