Thanks @billwear. So I did track the source of this problem down.
I installed MaaS within LXD per the docs here: link.
In that installation procedure, the example given uses the default lxdbr0 bridge which is managed by LXD.
The managed lxdbr0 bridge automatically employs dnsmasq. During container creation I named the LXD container “maas” (also per the docs), and so from within the container, and on the container network from test machines, dig maas +short was resolving maas, but it was being resolved by dnsmasq for the LXD managed bridge rather than maas DNS itself where the full fqdn was maas.lxd rather than maas.maas.
I had to stop using the managed lxdbr0 bridge altogether to get maas DNS resolving properly. I also found this post helpful to migrate away from the lxdbr0 bridge and onto a non-managed bridge setup from within the host OS:
https://discuss.linuxcontainers.org/t/lxd-via-snapd-disable-dnsmasq/450/2
If this issue and the other one I ran into regarding a failed maas install on LXD (link) were mentioned on the LXD install docs page (link), it would have saved me several hours of troubleshooting. It would probably save other LXD newbies substantial time too, so hopefully the docs there can get a minor update.
Thanks,
John