As noted in #1139 (comment) the current /etc/fstab config (added by common/conf/nfs) is not surviving the ISO install so is not working as intended.
I will be manually removing the fstab in the LXC build (bt-container in buildtasks) to resolve #1139 but we may need to consider how we might make this work as intended, while still supporting nfs within an LXC container. Note that nfs DOES work (with some tweaks) within an LXC container. See https://forum.proxmox.com/threads/is-it-possible-to-run-a-nfs-server-within-a-lxc.24403/