NXDomain reply after LAN IP response from forwarder for zone

I'm setting up a private VPN containing mobile devices and the home's LAN through a firewall part of the VPN network, and I'd like to use the FQDN of a LAN device and get the response from the home firewall.

So on my phone—part of the VPN—I configured it to use the IP of the firewall, then had a thought: it'd make more sense for the VPS to be the first to reply if it can to avoid additional latency waiting for the firewall to reply. The VPS is in a data center after all with faster links to public DNS servers compared to trying to go down a residential link for every DNS query.

So the plan is BIND on the VPS to use public resolvers, **except** when it's being asked something under the home's domain (this example: my-home.net.lan) where it should ask the home firewall for the IP. After some tries, this is what I have:

```named.conf, generated by nix
include "/etc/bind/rndc.key";
controls {
   inet 127.0.0.1 allow {localhost;} keys {"rndc-key";};
};

acl cachenetworks {  127.0.0.0/24;  ::1/128;  10.0.10.0/24; };
acl badnetworks {  };

options {
   listen-on {  any;  };
   listen-on-v6 {  any;  };
   allow-query-cache { cachenetworks; };
   blackhole { badnetworks; };
   forward first;
   forwarders {  9.9.9.9;  149.112.112.112;  2620:fe::fe;  2620:fe::9;  };
   directory "/run/named";
   pid-file "/run/named/named.pid";
   dnssec-must-be-secure xmit.net.lan no;

};

zone "my-home.net.lan" {
   type forward;
   forwarders { 10.0.10.101; };
};
```

And we try to resolve the home's firewall using it's FQDN from my phone over cell, watching `tcpdump` at the VPS:

```
IP 10.0.10.4.59410 > 10.0.10.100.domain: 51824+ A? firewall.my-home.net.lan. (39)
IP 10.0.10.100.30301 > 10.0.10.101.domain: 684+% [1au] A? firewall.my-home.net.lan. (62)
IP 10.0.10.101.domain > 10.0.10.100.30301: 684* 1/0/1 A 10.96.48.1 (66)
IP 10.0.10.100.domain > 10.0.10.4.59410: 51824 NXDomain 0/1/0 (114)

```

It's returning NXDomain, even though it just got a response from the firewall. Other queries (google.com, etc) work fine by querying the public DNS resolvers first like I want. What's going on?
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