@Wyatt I'm documenting a complete reproducer here, since you will need it for further debugging anyways and will also provide the logs:

MAAS + External DHCP + iPXE (LP #2069095 Reproducer)
----------------------------------------------------
A. Deploy an Ubuntu server to act as the DHCP + PXE server (I picked Jammy).
B. Disable MAAS DHCP
C. Configure your router to use this machine as DHCP Relay
D. SSH To this machine and follow this process:


(Disable systemd-resolved)
$ sudo systemctl disable systemd-resolved
$ sudo systemctl stop systemd-resolved

(Set Manual DNS- Use Your own DNS Servers)
$ sudo unlink /etc/resolv.conf 
$ sudo vim /etc/resolv.conf

nameserver 127.0.0.1
nameserver 10.1.10.10
nameserver 8.8.8.8

(Update / Install iPXE)
$ sudo apt-get update; sudo apt-get dist-upgrade 
$ sudo apt-get -y install ipxe

(Create the tFTP dirs and deploy binaries)
$ sudo mkdir -p /pxeboot/firmware; sudo mkdir -p /pxeboot/config
$ sudo cp -v /usr/lib/ipxe/{ipxe.efi,ipxe.pxe} /pxeboot/firmware/

(Create an iPXE Config File - 10.1.9.12 is a Rack Controller)
$ sudo vim /pxeboot/config/boot.ipxe 

#!ipxe
chain http://10.1.9.12:5248/ipxe.cfg-${net0/mac} || chain http://10.1.9.12:5248/ipxe.cfg-default-amd64

(Tree View)
$ tree /pxeboot/
/pxeboot/
├── config
│   └── boot.ipxe
└── firmware
    ├── ipxe.efi
    └── ipxe.pxe

2 directories, 3 files


(Install DNSMasq)
$ sudo apt-get -y install dnsmasq
$ sudo mv -v /etc/dnsmasq.conf /etc/dnsmasq.conf.backup

(Configure DNSMasq)
$ sudo vim /etc/dnsmasq.conf

# This is designed to replace MAAS DHCP
interface=bond0
bind-interfaces

# Match MAAS Domain
domain=alannet

dhcp-range=bond0,10.1.13.100,10.1.13.200,255.255.248.0,8h
dhcp-option=option:router,10.1.10.1
dhcp-option=option:dns-server,10.1.10.10 # Main Local DNS Forwarder
dhcp-authoritative

# Enable logging
log-queries
log-dhcp

# Set tag "MAAS" if request comes from iPXE ("iPXE" user class)
dhcp-userclass=set:MAAS,iPXE

# Enable tFTP
enable-tftp

tftp-root=/pxeboot

# Boot config for BIOS systems
dhcp-match=set:bios-x86,option:client-arch,0
dhcp-boot=tag:bios-x86,firmware/ipxe.pxe
dhcp-boot=tag:MAAS,tftp://10.1.8.44/config/boot.ipxe

# Boot config for UEFI systems
dhcp-match=set:efi-x86_64,option:client-arch,7
dhcp-match=set:efi-x86_64,option:client-arch,9
dhcp-boot=tag:efi-x86_64,firmware/ipxe.efi
dhcp-boot=tag:MAAS,http://10.1.8.44/config/boot.php


(Enable / Restart DNSMasq)
$ sudo systemctl restart dnsmasq
$ sudo systemctl status dnsmasq
$ sudo systemctl enable dnsmasq


(To work around LP #2002303 we need a nginx + PHP server)

$ sudo apt-get -y install nginx php8.1-fpm
$ sudo systemctl enable nginx
$ sudo systemctl enable php8.1-fpm

$ sudo mv /etc/nginx/sites-available/default /etc/nginx/sites-available/default.backup
$ sudo vim /etc/nginx/sites-available/default

server {
        listen 80 default_server;
        listen [::]:80 default_server;

        root /pxeboot;

        index index.php

        server_name _;

        location / {
                try_files $uri $uri/ =404;
        }

        location ~ \.php$ {
            include snippets/fastcgi-php.conf;
            fastcgi_pass unix:/run/php/php8.1-fpm.sock;
        }

        location ~ /\.ht {
            deny all;
        }
}

$ sudo vim /pxeboot/config/boot.php 

<?php
parse_str($_SERVER['QUERY_STRING'], $output);
$mac = $output['mac'];
if ($mac) {
	$raw_ipxe = file_get_contents('http://10.1.9.6:5248/ipxe.cfg-'.$mac);
} else {
	$raw_ipxe = file_get_contents('http://10.1.9.12:5248/ipxe.cfg-default-amd64');
}

echo(str_replace("nomodeset", "nomodeset initrd=boot-initrd", $raw_ipxe, $count));
?>


$ sudo systemctl restart nginx
$ sudo systemctl restart php8.1-fpm

Done!