The original DNS forwarding syntax from the dnsmasq days was listen-on <intf>. That allowed the user not to think about the addresses of that interface, which arguably was a good thing at least in some situations.

pdns doesn't allow that, so configs have to be migrated. What we do now is retrieving the address option from the config for the interface from the listen-on option. The problem is that there are interfaces that use a different syntax or may not have an explicitly configured address.

One obvious example is OpenVPN that may use local-address option rather than address, and doesn't have to use it. OpenVPN server interfaces often derive the address from the server subnet, so there's no way to migrate them in general case.

Luckily, this is unlikely to break anyone's network, and it's relatively easy to update the syntax by hand, so it should be documented as a breaking change.