TrueNAS isn't a supported OS.
I'm guessing you are using the TrueNAS Pi-hole app, which is neither developed nor endorsed by the Pi-hole team.
Indications are that Pi-hole app would just be some kind of wrapper around Pi-hole's official Docker image.
Note that any Docker container environment variable value that you'd pass into a container would become read-only within Pi-hole, potentially explaining your observation.
In your case, the associated variable would be FTLCONF_dns_listeningMode.
To change it, you'd have to edit your Pi-hole container's environment variables through your TrueNAS app's UI (which may look similar to Name resolution of locally significant names works except for Pi-hole's own hostname - #9 by airflow2010).
As we've released Pi-hole v6 last week, it would also be quite possible that the TrueNAS Pi-hole app may pull a new v6 image while trying to apply old v5 environment configurations, which would fail.
If that would be the case, this would have to be addressed by TrueNAS app developers.
(Or you could try to fix that yourself by adopting v6 environment variables, see Upgrading from v5.x - Pi-hole documentation - provided the TrueNAS app would allow to add and remove variables as required.)