Found the answer. Sort of. I still don’t know where the MAAS Proxy system logs errors, so I don’t know the exact reason that MAAS was dying, but I did find the solution.
The answer is simple. The LXC container needs to run in privileged/nested mode to support MAAS. This isn’t the recommended way to run LXC for a public facing server, but if you’re running in a secure environment, it’s sometimes necessary. So, here’s the fix:
lxc stop {maas-container}
lxc config set {maas-container} security.privileged true
lxc config set {maas-container} security.nesting true
lxc start {maas-container}