Afaik this is an Ubuntu thing. Ubuntu uses netplan to manage the network settings, so if you change your resolv.conf directly, it will be overwritten by netplan after a reboot. If you want to change the DNS servers, you have to create a yaml file in /etc/netplan, or edit the existing one.
Note: It’s also possible that there is a cloud-init config file on your server that passes DNS settings to netplan. If that’s the case I don’t know whether the cloud-init config or your custom-config will be used.
But none of that should actually affect Mail-in-a-Box directly, and I’m pretty sure Josh and the contributors of MiaB have taken this into account, when they wrote their scrips.