Here are the differences between two identical CTs, one that has problem with Bind9 (vm01) another that runs fine (vm02). Both vm01 and vm02 are running Debian 9, on vm01 I installed Virtualmin/Webmin whereas on vm02 ISPconfig.
$ lxc config show vm01 --expanded > /tmp/vm01.txt
$ lxc config show vm02 --expanded > /tmp/vm02.txt
$ diff /tmp/vm01.txt /tmp/vm02.txt
4c4
< image.description: Debian stretch amd64 (20200121_09:09)
---
> image.description: Debian stretch amd64 (20191019_05:24)
7,9c7,9
< image.serial: "20200121_09:09"
< volatile.base_image: 594467907e3a5583689e8d9e867eebbc74037561526ab5fe46cfc989989de893
< volatile.eth0.hwaddr: 00:16:3e:22:33:11
---
> image.serial: "20191019_05:24"
> volatile.base_image: 60def082b85d4d5bf60e76a1d9f9640b378a35028a2cb3af6f57e852df4ff866
> volatile.eth0.hwaddr: 00:16:3e:22:33:12
$