DS-Lite means full ipv6 access, but ipv4 is NATted by the carrier. If you try to access it from a network that only supports ipv4, the carrier’s NAT will probably block it. If your internal DNS points to the external IPv4, that might cause the problem.

In scenario 1, the server is technically a router between the 11 and the 10 subnet. There is a sysctl that enables forwarding (i’m on the phone and can’t look it up right now). This must be enabled.

In your second scenario, the server appears to regard its subnet 11 address as just another address it has and replies.